I think it's larger than fandom. You can see this in all sorts of groups where hypocrisy and group think end up making people hypocrite to what they tout to teach and preach and lectures others about. Religion is an obvious example- Social movements is another. "My group is about kindness and inclusiveness, so I'm gonna fuck up others I don't agree with".
Fundamentally, human beings are lazy, self centered and have an ego that is mostly about being understood and concerned about ones own feelings than others. Even if other people suffer immensely on my expense (indirectly), me not being uncomfortable is my chief concern.
It strikes me as childish that one would think that there are only one way for people to be attached to something. The idea that if the alt-right enjoys pizza, you should not enjoy pizza because pizza is co-opted by the alt-right is profoundly stupid. That what you enjoy (like R&M) overlaps for other reasons with morons who enjoy the same thing is really not a reason to disengage from the activity.
There are idiots who use South Park as their baseline for political ideology. Okay- What does that have to do with everyone else? It knee jerks me ever so hard, to see leftists piss their and acting reactive to whatever deplorable people are doing. Every time they are doing something, a good upstanding progressive have to act in accordance and be concerned about their public image.
Stand by your fucking convictions instead of categorizing yourself and others into boxes. Fandom or ideology. You're not it. It's just something you have in common with. You're not an atheist. You're not a gamer. You're not a feminist. You're not a Game of Thrones fan. There is nothing of value to derive from you summarizing yourself as being "it". You're none of those things, and what you are, doesn't fit into any fucking box that can be ticked, because you're a walking contradiction of various thoughts and your personality is basically stitched together by biased memories and habits.
We tell ourselves we're this or that we are that, to give ourself and others, a way to feel like we are understood. It's not tone policing, as the way we speak, really give us insight into our own flaws and blatant hypocrisy. We live and breathe the same bullshit in other forms, that we look down on others for. It's just in different contexts. And we're very much unaware of it a lot of the time.
So it's easy for us to shit on fandom and make fun of these people, but most of us are guilty of this in different contexts. Particularly political (here on OT GAF).