It’d be really nice if users we have blocked would be unable to join lobbies that we’re not personally hosting. There are so many troll/unhelpful/toxic/selfish users that continue to spread a negative experience in this game and even if the host doesn’t have them blocked, we deserve to be protected from them nonetheless. Their being able to join can immediately sour a lobby because we know they’re going to make things difficult and we have no guarantee the rest of the lobby will notice/react to their behavior and rally a successful votekick.
There’s also the matter of trolls seeking revenge against a votekick by repeatedly trying to join and blast the team with a powerful Friendly Fire weapon and the Host might not know how to block in steam or due to the chaos of this game; be unable to open up their profile and Block them safely in time. If one user can block, it should be guaranteeing the safety of the lobby.