Still not a perfect solution, but AFAIK, it's also possible to pass iMessage authentication with a hackintosh. So the same spam vector is technically still open, albeit with a bit more work.
Hackintosh user here, you can get iMessage working but it takes some effort. You have to choose a serial number that seems plausible to Apple given other information, and then also hope you get a bit lucky.
I really don't think this would be a useful outlet for a spammer trying to set up iMessage accounts en messe.