It totally makes sense what you are saying. But there is a technical reason behind it. The system doesn't know you are a VIP member before logging in, only AFTER that. After logging in, the promise is kept.
Still I understand your annoyance, and this technical reason is not meant as a justification. What you could do, is check the checkbox "Remember me" and have your browser keep the cookie. That way, you remain logged in and stay free of ads. This is how I do it.