Can you not? In the relationship between the user and the relay, who is providing a service to whom?

Are nostr users consumers or content providers?

Should YouTube ask users to pay for hosting videos? Turns out that's not how it works, instead YouTube is paying its (contributing) users.

Twitter/X/Elon were also at least contemplating paying users for Tweets.

Remember your relay is useless if it doesn't carry notes worth reading.