Realistically, whoever is the admin of the instance can decide to block another instance for any reason, or no reason at all. Admins of threadiverse sites are maintaining (and owning) these instances as a hobby. If they decide they don't want to look at content about penguins during their leisure time, they can just ... block them.
Since threadiverse is a bit less mature than the mastodon ecosystem, there aren't any "big" democratically owned and managed instances, so most people are stuck with benevolent dictator for life situations.
Edit: Also, if an admin doesn't want penguin content stored on their servers (which they pay for), it's a bit strange to say they must store content they don't like on what is essentially their personal machine.