

Not a lawyer, but the usual explanation is that these contracts are “at will” and the contract includes language indicating it can be terminated by either party for any reason at any time.
Again, not a lawyer and I agree this is terrible behavior. However, this is why some people say you should never give notice until you’ve literally started your new job.
I think it’s partially due to the relatively small population and quantity of posts. Anecdotally, I never browsed r/all, but that’s my primary mode on Lemmy. I usually don’t notice the community a post belongs to unless someone brings it up.
Maybe that will change with time.