Pre emptive blocks are allowed, although they're sometimes controversial. Some admins are friends with admins on other wiki and will share intelligence with one another, and may block sockpuppets who are known to sockpuppet on each wiki, for example.
So overall, no it's not unheard of, but it can be seen as controversial.
For something to be against the rules, you need to find the rule. If you're asking if it is against FANDOM's Terms of Service, the answer is there are no parts of the ToS that prevent an admin from blocking someone for no reason at all.
All that said, when there is a claim that a user is blocked for no reason, but we don't know the name of the user, that claim is usually not to be trusted. However, if we know the name of the user and the wiki, we can look at the block log and confirm that at least no reason was given. For now however, I don't trust the claim.
Some wikis agree to be Affiliates. Others might call themselves Sister Wikis. Sometimes these agreements between select wikis include a "Three Musketeers"-like reciprocal "Blocked for One, Blocked for All".
Some admins simply watch the activity of a user and if they see them acting TRollish or vandalizing one or a few wikis—especially in Central—they decide to preemptively block them from theirs.
The Rules of some wikis even explain this as the case.
As already mentioned, it's not a rule to be found in ToU, so it is not a violation. After all, Staff allows Wikis to run themselves as they see fit.