I've got a feeling this is specifically related to DRM in the HDMI spec that prevents video capture of encrypted content. Maybe I'm remembering something vaguely from about a decade ago about HDMI content encryption that is no longer relevant, but my hazy memory is that this was a core element of the HDMI spec that media corps wanted to prevent digital copying. Not that it really means anything at this point, the seas are full of high quality rips regardless, but maybe there is some dubious legal value in preventing an open source driver?