Because there are no instances. There are relays that you post to, and that people use to fetch notes from. But there are no “user accounts” on the relays. If the note is signed by your private key, it comes from you, regardless of how it came to me. It can be through a bunch of relays.

Relays can and do filter notes by pub key. To fight spam, and problematic content. But you as a user can always change the relay set that you post to. And, of course, host your own relay, which is pretty straightforward.