Why is the boonie cap so demonized in the army?

I wore mine in Iraq! It’s functional in the desert! Those that didn’t like it (in my opinion) just didn’t like it so they wouldn’t let anyone wear it!

Absolutely true, in 1995 my Guard unit went to jungle school, when we returned our BC said we could wear our boonies in the field, someone complained about it so of course that whole thing was stopped

One of the first bad experiences I had on deployment was an SFC who asked my about my boonie. As he was dressing me down verbally, I mentally dressed down just how pathetic he was to think that headgear had anything to do with our mutual mission. It's the most useless use of rank I'd seen to that point.

We were issued them in the Fall of 1969 while in the Big Red One. I much preferred it to the awful standard issue soft cap. Westmoreland’s cap was not standard issue and was sharp. Marine 8 points are the best. I guess the modern army’s trying with its issue of berets which is an improvement over the uniforms we were wearing in the 60s. The pinks and greens are great. I almost want to re-up.

I'll take a guess and say it's simply because it's more informal looking vs other headgear. That's about the only logical reason I could think of.

When worn correctly the bonnie cap offers far more protection against the sun than a patrol cap. To this day I still wear mine from Jungle School (OD Green) and Desert Storm (old chocolate chip one). I did read CSM post citing too many customizations making it easier simply to disallow them which makes sense; however, they were created for a reason and one could argue that there is a great deal more sun exposure when in garrison than while utilizing concealment in the field.

Wore them in Vietnam.

Well like many have said, Sr NCO's sometime have nothing to do and want stir up shit for the sake of stirring up shit. The boonie hat has its place in the operational spectrum, i.e. for specific mission environments. Wearing this headgear in normal duties not in an environment that it was designed for is pretty much pointless, unless you are trying to impress the girls. In garrison or on weekend drill duty is not the place for that, the patrol cap is a far better choice, but then you have the beret issue which in my personal opinion was the dumbest move ever.

If it's authorized and issued no problem

I have no idea. In Vietnam, I'd have LOVED to have had a boonie hat, but alas, I was with a Marine unit that didn't have them and I don't recall ever seeing anyone (in that unit) wear one.
If for no other reason, they provide protection from the sun. Today, I keep having to have pre-cancerous moles or marks removed. In battalion rear areas, we wore either the USMC ball cap, OR went without headgear alltogether.
So what do I wear when working the acerage on my property? A BONNIE HAT!