"It's that they're internalizing the overall attitude that the writers display through the show's writing." Of course, that's South Park's "artistic" style. It's what South Park's audience wants, and that's okay.
The idea isn't that people are taking South Park's in-universe parody moral lessons seriously (though as Nevermore points out, you can't safely assume that isn't also happening). It's that they're internalizing the overall attitude that the writers display through the show's writing.
So many people who think police and court procedure works like it does in Hollywood, and act accordingly. Sometimes to their own disadvantage, sometimes just to the frustration of well-meaning authorities.
With Brian, he stopped being the voice of reason and started to be the right wing notion of a liberal, someone who's lazy, pretentious, doesn't work at anything, is every bit as abusive as the rest of the cast and only wins arguments with strawmen.
Is it because they changed or we changed? Stuff that was charming, or at least, funny at first can get really annoying the longer you know a person... and I find that can go for fictional characters as well.
"Here's a little---whatever. A little thing. A disclosure. An insight. Revisionism---I've always been fascinated by it. By the idea of a writer going back and altering things. Make no mistake: an edit is a profoundly aggressive act. As Froid once said, we grieve for the murdered word. If you want to get the measure of an author, don't look at what they've left on the page.... look at what they've taken away." -- Rung (MTMTE #28)