There was a blog post here recently where a repair technician with his own shop was trying to contact Apple about reportedly stolen iphones being reactivated and resold and it undermining his business. They then found more and in one case it was a rogue Apple employee doing the activations, and in another it was software tools they found and send to Apple, which got ignored for more than half a year.

I can't seem to find the original article but here's Louis Rossman explaining the same thing. The argument boils down to the fact that Apple doesn't care, more iphones means more people with wallets attached to them.

Also, it wouldn't necessarily be public knowledge on exactly how it's done, otherwise 1 Apple would try to fix the issue or 2 there's money to be made in selling the service.

Edit: typo