You're correct on that. If every relay you connect to censors Bob, then you won't see anything from him.

Bob and his followers have a lot of options other networks don't have to check this is happening. For one, Bob can spin up a new npub, DM a lot of his followers to tell them the censorship is happening, signed so they know for sure. Some will verify, and once the word is out new relays get spun up and old dies. Bob's regular buddies can track it too. And security focused groups can regularly audit relays, and easily sound alarms.

So long as some people are vigilant against abuse, and the rest are willing to change their relay setting when the alarm is raised, bad relays are easily abandoned because none of the data they host is exclusive or under their control.