What's going on behind the scenes is that very few people don't use an email address with one of a handful of TLDs, and that restricting signups to a known-good

What's going on behind the scenes is that very few people don't use an email address with one of a handful of TLDs, and that restricting signups to a known-good list solves some support and a lot of abuse problems. Not saying it justifies it, but that's for-sure one reason places do that.