They could. But they don't, (not mine anyway). And they'd be booted as soon as they start spamming.
They WILL, but then, it's a war :-)
Discussion
I see, so the paid relay can kind of surveil a little bit? Optionally.
ALL relays can surveil. The free ones are trying ways to combat spam. Asking for money is effective. Later, relays will implement local policies. Disney AUP will be different to Playboy or TrumpTown.
Nostr doesn't censor, because anyone can run a relay. But relays can, and will censor. If only to stay legal in their location.
Pretty fkkin elegant! 🎩