Why is the boonie cap so demonized in the army?

Check out the headgear in my photo and you can tell how this will go. The idea of the boonie hat was to increase camouflage while at the same time providing increased protection from the sun as well as visibility to the wearer. That said, it could not be pressed at a time when all uniform items were to be starched, so it looked 'unprofessional.'
It was fairly standard in the fileld when I was in the Marines in the 1980's, and was part of the normal headgear when I was in the Army (Guard under 3 BDE, 101st Abn) in Afghanistan in 2010.
I do agree with the 'Busy Body,' nature of some of the Sr. NCO's. I heard that Command SGT Magor at Bagram became a stiller for turning people away from the chow hall if they did not have eyepro and gloves on at the door.
They say that 'cream rises to the top.' But it is also true of floaters in the crapper.

I love boonie caps. They are so practical shading my eyes, neck and ears from the sun that conspires to produce skin cancers on my head, Besides, in the rain, the boonie keeps the runoff from running down my neck. Boonies Forever!

They are in love with their colorful berets. Why anyone would adopt anything with ties to the French military is beyond me.

First thing you have to do is separate the Army into it's proper groups. Combat, Combat Support, Combat Service Support, etc...
Then you can see where the disconnect between fashion and function starts. I have been on deployments where troops weren't allowed in the chow hall without shined boots. That's what happens people in leadership positions (notice I didn't say leaders) get bored.

Just to let people, who don't know (like Air Service folks, like me), this is a boonie cap.

Just like what pretty much everyone else has said. Plus, 25ID has something to differentiate themselves with like other Airborne units do with their maroon berets. All my deployments, leadership always flip flopped when the boonie was authorized. I remember one time down range, the newly appointed regional SGM had common sense and even said it was stupid to have to wear eyepro everywhere you go and no longer made it a requirement. Now, if it was sunny outside, yeah sure. Use common sense and put the dark lenses on. The best part is when he mentioned that if you took IDF, is eyepro going to save you from a motor round to the face or 7.62x39 into your eyeballs? Obviously not. The fact if the matter is, in the military, if it makes sense, it's probably not authorized. That's the culture we need to help change.

I wore my boonie on my two deployments in Iraq as it was the only piece of headwear that protects both the face, ears, and neck. The truth is it is senior NCOs that have nothing better to do than enforce a standard. A standard in their own head.

Most likely your battalion or company commander decided that wearing of the boonies cap wasn't warranted for the field exercise. This decision should have been briefed before heading out. Can you quote the AR regulation covering the wear of the boonies cap?

Yes. A frustrating topic. It was especially exacerbating right after 9/11. The big Army recognized short comings in field gear and issued stuff to soldiers in order to help them cope. Unfortunately, updates to AR 670-1 came out slower than the equipment. The big ones that seemed to really piss off senior NCOs were black fleece, boony hats, and ballistic sunglasses. I remember having the following argument with a senior NCO about new boots that were issued. “Sir; you can’t wear those. They aren’t in AR 670-1.” Me: “But the Army gave them to me. They must want me to wear them.” Senior NCO: “Sir, they aren’t in AR 670-1.” Me: “Yes sergeant major. We established that. But the Army issued them to me.” Senior NCO: “Sir, but they aren’t in AR 670-1 so you can’t where them.” At that point a quote about arguing with idiots came to mind.
The one that really blew me away was the ballistic glasses. I was going into a chow hall and there was a poster of a soldier with his face pockmarked with shrapnel, including his ballistic glasses. Which saved his eyesight. The caption read, “NCOs, make sure your soldier always have their ballistic glasses.” Meanwhile, a 1SG was inspecting soldiers coming through the chow hall lime for uniform violations. Among other things he was telling soldier to take off their ballistic sun glasses. If they were in your pocket they were probably going to break (as several of mine did), and you wouldn’t be in a position to get them on.
The boony hat wars the same. Provided extra protection for soldiers, albeit against the sun, not shrapnel. But senior NCOs didn’t care.

It's entertaining to read the posts that go back and forth over such trivial matters... two words, "ARMY"!