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 think it's good that it's a simple flag and not an outright block.
It's just a way for admins to see "maybe take a closer look at this registration."
I'm sure there are a lot of legitimate users that use these types of emails, though.