Spammers, trolls, and ban evaders often use temporary email addresses. PieFed now checks a list of known temporary email providers and displays a warning icon next to registrations that use such services.
If registration mode is set to "Open" (no approval needed) then the site admin(s) receive a notification instead.
A throwaway email address isn't always a bad thing but it's one factor that admins might want to take into account.
I don't really understand the hostility to this. Do users understand why emails are part of account registration in the first place instead of letting you sign up without an email?
Because people forget passwords and demand a password reset mechanism. Also a place to send terms and conditions and changes to those.