Behavior like that needs to come with a huge warning label.
It would be trivial for any man-in-the-middle to block https and server http.
It would be trivial for any man-in-the-middle to block https and server http.
The main problem is preventing downgrade attacks. With mail it is easy to just remember the setting for every server. Not so with websites.
[1] https://www.agwa.name/blog/post/starttls_considered_harmful