Perhaps it was felt that it made it too easy to doom huge swathes of the internet to being unable to e-mail you.

I can see the situation of someone receiving a spam e-mail ending hotmail.com, or aol.com (and probably spoofing that as well.) and user adding the domain to blocked senders.

End result - user doesn't receive a whole stack of mail they should have, and probably - knowing the tortured logic most people adopt when things don't work as they thought they would - they blame MS. But worse, of course is they may never find out, and miss out on business because of it.

Anyway, given I have absolutely no insight that's just a guess.