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 know which service you use or whether it is on the list. But if it’s a paid service then it’s probably not on the list. Things on the list are like https://10minutemail.com/ or https://www.guerrillamail.com/
You’re free to maintain your privacy and admins are free to weigh the potential risk of accepting an application with a warning icon on it. I’m sure there will be plenty who ignore it.