The "paid" aspect is seemingly not the problem. Automattic does what Beeper does (without unauthorized access to Apple services) and charges $15/month. https://texts.com/subscription
It looks like their service only works on MacOS, which means they're probably just using the local iMessage db like others have done. They're not integrating with the API or providing access for other platforms, it's a totally different (and limited) solution.
If you have a Mac sitting around, airmessage.org has made this work cross platform for a while now.
My feeling is that Beeper leadership saw the kid's hack and adopted it without thinking it through. They could've designed it in a way that didn't depend on unauthorized access to Apple services.