"surely the GPU manufacturer already paid the royalties necessary to implement and use the codec"
Codec patent licensing is a complicated topic which often defies logic. Your GPU manufacturer may not have paid a licensing fee or at least one that covers all use.
Licensing fees are required by whoever enables the full encode/decode path. Your GPU may provide acceleration, but if software is required to enable full path encode/decode the software which uses the acceleration is required to pay the licensing fee not the GPU maker.
If you are purchasing a consumer GPU and the manufacturer has paid a licensing fee it is likely their licensing fee only covers personal consumer use of the codec. If you use the codec outside that scope either yourself or the software which enables the full decode path are responsible for acquiring a license.
