Why is Collection not simply treated as Collection<?>

Question

Consider the following API method taken from Shiro\'s org.apache.shiro.subject.PrincipalCollection interface but probably present in other libraries as well:

Answer 1

A Collection screams:

Please don't add anything to me. I have a strict content type, ... well uh, I just forgot what type it is.

While a Collection says:

It's all cool ! You can add whatever you like, I have no restrictions.

So, why shouldn't the compiler translate Collection to Collection ? Because it would put up a lot of restrictions.