Individual instance owners can do literally whatever they like.
Put up ads.
Charge a subscription.
Anything.
Let’s say instance A is hosting a community that everyone on instances B and C love to participate in.
But A want’s to earn some money so they start charging access to their local users.
This doesn’t effect users of B and C at all, because the data is federated.
The owner of A get’s grumpy and defederates B and C.
The users on B and C find somewhere else, either on one of their instances, or on D.