If I understood the issue correctly, the servers that you played on, are Non-Premium, which means that you can log on without an account. Servers like these need to have a Login plugin installed to prevent someone logging into another persons account. The login issue is same for every player that's on the server, even if that's a Premium player. The reason is that the server can't magically detect if you are a Premium player (atleast to my knowledge), so it just throws the login screen at you, because anyone could just set their username to yours and play from your account (on the server ofcourse).
If I understood the issue correctly, the servers that you played on, are Non-Premium, which means that you can log on without an account. Servers like these need to have a Login plugin installed to prevent someone logging into another persons account. The login issue is same for every player that's on the server, even if that's a Premium player. The reason is that the server can't magically detect if you are a Premium player (atleast to my knowledge), so it just throws the login screen at you, because anyone could just set their username to yours and play from your account (on the server ofcourse).