I don't think it's legal issue so much as a violation of their own contract (and no, I don't think "we reserve the right to remove anything for any reason" is very binding. Just a CYA for if/when they DO actually break the law by removing content that could get them sued).

There's nothing saying you can't talk about other communities, and there is in fact an entire sub focused on "reddit alternatives" (guess the name). So for them to ban accounts for this without a clear change in rules violates that trust. But ofc coporate interests and trust are nearly parallel at this point.