There is absolutely nothing preventing AMD from delivering their own open-source drivers to their customers.

They want it in kernel, however, and that means they have to play by the in kernel rules.

AMD didn't deliver a Linux driver. They delivered a Windows driver plus a bunch of glue code that nobody really wants. In the best of all possible worlds, somebody would dissect the Windows driver for instruction and write a proper Linux driver and nobody would try to run the Windows driver as-is.

This wasn't an open source driver, this was a hardware abstraction layer that they wanted in the kernel so their Windows driver could be distributed under a proprietary license and used without a kernel recompile.

A year ago AMD said they would just use what is currently in the kernel with a few minor modifications to get their driver working in userspace, apparently they did not follow through on that though