Because you recognize how freaking bizarre it is.
I really wish this kind of thing could be understood better by both sides about a whole host of things.
There is a huge difference between saying you
CAN'T do something like that and saying it's weird, or bizarre, or unsettling, or unusual to do something.
It's a certain level of societal tact and understanding, it's a level of internal introspection and concern and consideration for your fellow man, and it's a level of understanding that the world is not a movie/book with you as the starring actor.
Can you go walk out in the middle of the street with a AR-15 strapped to your back? Or with your ass hanging out of chaps? Or dressed in drag? Or wearing a "[Politician] is Hitler" sign? Or a confederate flag shirt in Massachusetts? Or having gauge piercings all through your face while dressed in all black?
Sure. You absolutely can.
But if you do it expecting people to just treat you as a "normal" person going about an "every day" routine that is not "weird" then you're frankly either being willfully ignorant or you're so egotistical and uncaring that you don't even have the ability to understand or empathize with society at large. Now if you go out, recognize you're going to be looked at as strange or weird or unsettling, and you accept that and the realities that such a thing causes...cool, more power to you.
None of this means you can't do it. Doesn't even mean you SHOULDN'T do it. What it does mean is you shouldn't do it and then act aghast or confused or insulted or indignant that people treat your odd and unusual public presentation as they would any other odd and unusual public presentation.
I'd say people need to use some common sense, but often that's not even the problem because the reality is that often times when people make it a point to be as brazenly outside the norm as possible in public, the kind of attention they get is specifically the kind of attention they WANT to get for one reason or another (whether it's to draw attention to themselves or a "cause", or to feed their desire to be a victim or "nonconformist", etc)