With forums there are arbitrarily many recipients of any message. A lot of people may not like furries or communists or what have you, but demanding that every other instance ban them as a prerequisite to interoperability is how you kill the network by suppressing everything but the lowest common denominator.

This problem is created by tying accounts to instances. Because then other instances, instead of banning the accounts they don't want from only their own instance, can try to get them banned from everywhere by threatening any instance that allows them with disconnection. Which is poison.