Having spent a few years focused on this (and other semiconductor enablement-related problems) my guess is they have evaluated their own patent portfolios and the risk of their implementations triggering a war, and have concluded it was worth doing. Note that Intel's GPUs are relatively simple, so AMD would seem to be bolder here, but I haven't dug into the detail to see how they have limited their exposure.

I find Intel is pretty solid about open sourcing everything they provide for Linux that runs on the host — it's a pity they haven't applied the same policy to the ME.