Spammers who bother with email based registration sites at all, often take the time to routinely acquire one address from a major free provider and use it everywhere they can't use a throwaway as well as get naive people to send email to it.

For other people who kind of don't trust your site, its future, or its security, but are too lazy to create extra accounts, you are forcing them to do a full evaluation in a way that adds additional weight to the possibility the site is an actual data broker today instead of an existential spam threat.

Personally, if I decided a site was bellow the threshold for my real email on an initial encounter and then I saw it perform this kind of detection, I wouldn't touch the site again.