Isn't this a rampant GPL violation?

Eh, it could just be that there isn't sufficient overlap in the incentives of the vendors and upstream.

For example, an OEM can bash out low quality code which is hard to maintain, secure in the knowledge that they'll only have to maintain it for the year or two that phone is getting updates, and it won't have to survive any big kernel updates or be compatible with anything that isn't on the phone.

Upstream, on the other hand, wants to maintain device support forever so they want good quality code that they can get through kernel updates with a minimum of fuss, and they can't accept anything that breaks existing code.

So upstream won't accept the OEM's shitty patch, and the OEM won't pay for the engineering time to make patches that upstream will accept.