Wouldn’t that be the same thing with no other percentages in sight because we’re subtracting from 100%?

I don’t think so. It’s probably what keeps it small and more personal. There is also the notion of responsibility: if a person I invite causes trouble, it’s potentially on me. Maybe not on the first infraction, but if one invites 20 spammers/cryptobros/venturecapitalists, it’s reasonable to block the inviter too.

I’m not arguing one way or another (that’s not my decision anyway), but I can understand why they do this.

You’ll probably enjoy Lobsters: https://lobste.rs/