I think the reason why people complain about politics is:

• They want to find an easy scapegoat. They didn’t like the movie, and it’s easy to blame one aspect when the actual reason is far more complicated. It’s like how people think Jar Jar Binks ruined Episode 1 when he was just one small part of the movie.
• Politics can often be badly implemented in movies. Not all politics are black and white where there are obvious good and bad sides. The characters of Princess Mononoke, while it’s clearly on the environmentalist side, shares multiple perspectives on the conflict.
• It’s easier to notice something when it’s bad, so people tend to notice the politics when it’s bad.