Completely wrong. Just because this is a sporting event where beer is sold, that doesn't give anyone the right to check their manners at the door. Home fans, away fans, it doesn't matter. You are still in public, act like human beings. Would you walk down the street and tell people something they like sucks, or their city sucks, or tell them to go f*** themselves? No, so why do people think it's acceptable once they're inside?
It's so rare to find decency anymore, and this idiot mob mentality is a huge problem.
I wore a Cubs jersey to a SF Giants game last season, and the fans were totally classy and treated me really well. I got a couple jabs about how they haven't won in 100 years or whatever, but it was always with a smile so I knew they were just messing around. I never felt unwanted and I will go back and spend my money at AT&T Park as often as I can because the fans treat people with respect, it's a great place to watch a game, and I don't have to sit and listen to drunk douchebags tell me that the Cubs are this or that.