Of the themed instances that exist now, I'd be willing to bet that in addition to their local communities they host that they also subscribe to other communities that aren't strictly related to whatever theme they are going with.

For example, I'm sure the Star Trek instance also subscribes to the lemmy@lemmy.ml community so the admin can stay abreast of Lemmy news. And probably also follows other technology related communities as well.

I think most people would just want to gravitate to whatever they want to be identified with. There's nothing stopping you from joining a music themed instance and then adding some non-music subscriptions to your list. It doesn't force those subs on anyone else on the instance.

And if you don't want to be identified with any specific topic or community, you can always join a general Lemmy instance like Beehaw or Lemmy.world and subscribe to whatever you like piecemeal.

Why would I join an instance about music

Maybe the user feels music is an important part of their identity and likes the idea to call a music instance their home. Or any other reason, doesn't have to have any reason, especially not a reason which is compelling to others.

to end up subscribing to communities that have nothing to do with music?

Maybe the same user still has other interests besides music and likes to follow those.

Or in summary: Why not? It's possible.

For different people, different criteria will influence their choice of home instance. Some may even choose to have several home instances. Other factors might be uptime, latency, defederation status, size, local communities, rules, ...

For most people, it does not matter much what their home instance is, which is just another possible explanation for registering on a music instance and subscribing to remote, non-music communities. Like how you can register to Microsoft services with a Gmail address.