I've asked this 3 times across the various threads on this, and have yet to get a 100% confirmed "you are right" answer because I don't know the specifics and I haven't come into anybody who has taken the time to research the answer: When you log in to iMessage and the various underlying protocols/services, you do it with an Apple ID. Is that Apple ID tied in a one-to-many (but you need at least one) fashion of regis…
As long as the people using fake serials are a small group of hackers, developers, etc. who are hardcore Apple fans in the first place and merely building things for personal use, I can see Apple intentionally turning a blind eye.
But once that starts getting abused to build a consumer product at scale, it becomes a whole different story.
And I have to imagine it becomes difficult when dealing with old devices, 99% of which have long since been thrown out or recycled, and those serial numbers are ripe for re-use, but there are still people using 10-year-old iPads.