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?
To be fair Lemmy does allow you to sign up without any email at all (or at least used to, dunno if they changed it), so it's not surprising that this goes against many peoples expectations.
Pretty much every Lemmy instance requires email. The only ones that do not require it are small enough that spammers haven't found them yet.
Some require an essay application instead.