In Privileges, It says creating posts is a privilege limited with 1 reputation. Then If someone's reputation drops to 0, can't they create posts?
Reputation can't drop to zero. It never falls below 1.
Creating a post is a privilege, because it can be taken from you. If you post too many badly scored questions or you delete them there's a chance you won't be able to post any more.
Because reputation can't below 1 (as others noted) this is the only way to lose this privilege.