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).
The problem is, how else do I express my rich inner world to other humans - and how can they express themselves to me? Symbols, whether they be the words of language or pictoral images, are our primary mechanism to express our conceptual mapping of the world and the inferences we draw from it.
A more "accurate" term than saying "I am a member of [category]" would be to say "I possess or am actively cultivating certain characteristics or beliefs that are centrally codified as [category]" - because all objects within a category have a degree of centrality to the prototypic image of that category.
There is definitely the problem of our tendency to, when unexamined, gravitate towards the centrality of whatever categories we compose our outwardly appearance of - to model oneself after an archetype. The problem is when people substitute the maps of their identity constructed from others media for their own identity. Again, drawing inspiration from the world is great and can teach you a lot about yourself - but fully destroying your own agency in doing so loses something valuable.
But categorization has value in spite of our own poor usage of it. It allows us to make sense of the world and construct complex systems. It allows us to communicate far more effectively even if connotations sometimes muddy the channel.
Signalling through self-identification has instrumental value - presenting yourself in a clearly understandable manner is useful so as to guide your observer to drawing the conclusions about you that you desire. It can also signal how you are likely to feel towards a particular topic - or how safe a person with a set of traits can feel around you. It makes people feel more at ease interacting with you when they have somewhat of an understanding of you, and language helps to enable this understanding.
Basically, humans and their anthropocentric language can be confusing, but it's not surprising that humans have a natural tendenxy to use themselves as a frame of reference for abstract concepts.
(Also, refusing to self-identify and making a point of why you refuse to sometimes sends messages too, if there is a central commonality between those who do so - see people who really don't like the word "feminism")