It's because it is a common spam action to use <site>@<free_email> when blasting out stuff. It's also common to try and use <something>@<site> in either/or the

It's because it is a common spam action to use <site>@<free_email> when blasting out stuff. It's also common to try and use <something>@<site> in either/or the to/reply-to fields for spambots. So, it is easier to blacklist it altogether.