Why is the boonie cap so demonized in the army?

Because it is NOT part of the authorized uniform. I had to finally go to a Dermatologist to get a profile to wear one over 20 years ago. While at Ft Hood I had a full Colonel prescribe me a boonie hat to be worn outdoors every time I went out. An injury to my nose required a skin graft and I was already showing precancerous cells in my skin.
I had everything from E5’s to CSM’s demanding I take my hat off and wear the proper attire. I would show them my profile and most could care less about a peace of paper. These uniform Natzi’s were far more concerned with my uniform than they were with my health. I had to appear in front of my commander many times because I refused to remove my boonie hat. It took the actions of the same Colonel who wrote the profile to get involved and remind those little Natzi’s that no one short of the Army chief of staff can override a medical requirement per AR40-502. I was harassed, ostracized and even frowned upon by many within and outside of my command.
I retired after 32 years of service. In my entire 32 years my service to the Army and this nation was perfect. I never failed at ANYTHING I ever done and exceeded expectations at every job I had going above and beyond on every assignment. I was just diagnosed with skin cancer just over a month ago. I start Chemo in just over a week from writing this response. My question is this. Does any single person who thought the uniform code was so much more important than protecting my health cares one iota whether I live or die a horrible death as a result of BOT following that profile to satisfy their demand uniform code of LOOKING correctly? That of course is a rhetorical question because I think we ALL know the answer to that question. While I have the utmost respect and love for my uniform and my Military family there are those who believe following the rules to the letter is much more important than the health of individuals.

I've always let my soldiers wear them in the field and on the range, catonment was a no just so some SGM/CSM wouldn't get all pissy.

It's a good piece of headgear. While I liked the beret, I understand how it was more difficult to put on, especially if you had a handful of items you were carrying on some errand... or just during the course of daily duties. The boonie is a an even simpler piece of headgear to put on than the PC. And, as has been pointed out, it offers better sun protection than the PC. Considering that safety briefs routinely advise soldiers to not get sunburned because it's "damaging government property."
Likely, some senior leadership just doesn't like things that are different. As has also been pointed out, soldiers like to wear it in some unauthorized way. Rather than point out the regs (if they know them) regarding the boonie, they choose to throw a fit and make up regs or rules. Just like with the "Ranger roll," guys will try to do "cool guy" shit to the headgear. I guess for some, it's easier to just try not to allow a sensible alternative. Funny how "knowing is half the battle" was G.I. Joe's thing. I guess more senior NCO's are about the power and asserting it than educating troops or instructing them to get the proper instructions from their leadership.

Senior staff wore starch jungles, full color patches. They were not in the bush, so they disliked what worked for the guys who did the dirty work. Now things are lighter softer and thankfully troop friendly. Different time different military. Thankfully everyone can now serve! To all stay safe stay strong and glad there are still 4%s who serve America!

In the air force, we always got squadron baseball hats. Yep. So not only do we look like bus drivers (my husband was riding amtrack on leave and people kept asking him questions because they thought he was an employee) and women look like we're wearing a business suit, we get to weat baseball hats - like we're truckers or something.
I like the boonies hats for down range, but I'm not quite sure about h one station. They're much more uniform-like than a base ball hat, though and they don't show dirt and grime like the white front of the A.F. base ball hats. I really like the berets or just the standard clothing sales hat.

Anyone ever been in a jungle with at least two guaranteed downpours a day without one? A poncho hood sounds like you're next to a rock concert's bass drums. Comfort is preferable to couture. A boonie hat and a cravat did a lot to sop up the sweat when it was 100+ and 99% humidity. Also, without it, where would one put his decorative flechette bling?

It's a good question. It would be great for wearing during the summer. Sun is brutal in TX, LA, KS, KY, NTC but no, let's wear the beret so we have ring around the nugget and skin cancer. Don't do anything that is common sense. I guess some CSMs are just love being dicks.
We had came in from the field, it the wash rack, with tanks, and wearing a pair of nomex for a week. We were nasty. Got into formations to be released, still wearing our patrol caps. The BDE CSM was driving by, stopped and reamed out the 1SG bc we weren't wearing berets. And he made us go put them on before he left. What a cluster.
Same thing in Afghanistan. I was on a 6 man team of combat advisors to the Afghan Border Police in the Kunar, Nangahar Provinces and the Tora Bora region. Didn't have any issues with the PRT Asadabad but at FOB Fenty in Jalalabad, had a few heated exchanges from butterbars wanting to be saluted, our relaxed facial hair and boonies. It's was quite entertaining.
But there isn't any reason not to wear them. Someone mentioned about wearing properly, hell they couldn't wear the beret correctly either. Nothing but soup sammiches.

I spent 30 years in the Infantry, and have permanent sun damage in my neck to prove why the Army should allow Soldiers to wear the boonie as opposed to a PC. However, there is ALWAYS a counter-answer to everything like, “get sunscreen from the medics…”
It is what it is. And as another CSM commented already, it makes too much sense as these items are already in the U.S. Army clothing inventory NSN

I've worn a few versions of the "boonie" cap (desert camo, ACU, etc). I have never encountered anything negative from Senior NCO's nor Junior enlisted or even Officer's in my 28 years. Generally, the first field grade Officer/NCO prescribes the field uniform and garrison uniform or passes on the instructions to and from higher/lower HQ. Unless it was worn not IAW 609-1 etc I see no problem. I've worked with Joint Staff including 2 Star Marines, AF etc as their Army Sergeant Major. They usually deferred to me and I would issue a "may wear" . I was not SpecOps however I know they used modified uniform wear including hygiene such as a beard etc. My highest level was a CENTCOM command and there was never an issue there. It's unfortunate that there are those that demonize the head gear and I don't know why. (Insert a hundred speculations). Good question.

The only thing I can think of is that . They just need something to complain about.