My theory is that it's to dissuade less advanced spammers/fraudsters who use manually-created, consumer-grade email addresses that take time & effort to create,

My theory is that it's to dissuade less advanced spammers/fraudsters who use manually-created, consumer-grade email addresses that take time & effort to create, thus rejecting them could actually slow them down or piss them off enough to give up. The intent is probably to ban spam/free trial abuse or obviously-incorrect addresses (though for the latter case why not just send a verification email).