I don't think anyone's saying apple must allow them (though such people admittedly exist on the internet, in retrospect). I think its more of disappointment at the speed at which apple is "patching" this.
It seems the CEO of Beeper is literally doing that.
As an Apple user, I'm glad they're patching this fast. It's a security issue and privacy one. Sure Beeper may be using it in a non-nefarious way, that doesn't mean bad actors aren't gonna use this to spam the heck out of iMessage users.
You know, I have been getting constant scam texts from "Amazon" for a $289 vacuum that I'm supposed to call a number to cancel the order and it's coming from an email address, or a phone number. Never really got them before in that way.
I've been getting iMessage spam long before Beeper existed.