I suspect that, in many instances, people who do that are not trying to convince anybody of anything. If they expected to be greeted everywhere with agreement then they would not go around saying things which they know perfectly well are not likely to result in a vigorous and healthy debate if they were said to strangers in the street :)
From some of your other comments I gather that you've been around this material for a long time and you know a lot about it. Why not share some of what you know, so others can learn? and make your substantive points thoughtfully? Then you'll be making this place more interesting, and your points will have some persuasive power. Just venting only adds energy and credibility to the views you disagree with.
I get that there's a low probability this argument will work with you but I think it's worth trying to persuade people that it's in their own interest to follow the rules, which are designed to try to keep a community that's interesting for everybody:
I get that the two things have some overlap and aren't cleanly divisible. What we do is a major part of who we are.
But I mean, it's one thing to say "I think it [his architecture] is awful", or even "I think his architecture had a negative impact on people/society/cherished values/whatever" but quite another to say "I think he sought the ruination of everything good and decent because he was a demented and feeble mind." Because, yes, I do think the last one would only be appropriate if you actually knew something about the guy...
Edit: And yeah, of course, I'm not trying to censor anyone's opinions. Of course you can _have_ the opinion, you can even express it. I just think that it's not what engaging in productive/civil discourse looks like and, depending on the venue, people may call that out or whatever.