Yep. Banning users who aren't subscribed to a community and are just downvoting everything that comes up is pretty common and understandable.
The typical modlog entry I see for that is "block the community or curate your feed", and I think that's pretty justified. It's like intentionally showing up somewhere you don't want to be just to "booooo" everything there. Being asked to leave and not demoralize the people just trying to exist in their own space is perfectly fine, IMO.
Thanks. I'll add that to my test cases for the UI I work with. I don't think I've ever tried to block a community the test user was banned from, so I'm curious if it's an API restriction or the way the UI handles it. Will prob also submit a bug once I figure out which.
Edit: That looks like the community options that have the "block community" button are all hidden when you're banned rather than the "block" functionality being restricted. I'm almost positive now that it's just a UI bug. Will still add it to my test cases, though.