The .mat files are copyright Corel, so I don't see a problem. In fact I don't see how someone can upload a bunch of Bryce skies and say non-commercial use only because they can't claim copyright on them, so how can they restrict usage? Presets are presets. For example, Curious Labs claims copyright on their presets (lights, cameras, etc.) and state noone else can claim copyright though presets certainly can be distributed. The only problem I'd see with .mat files would be if they contained a 2D image, then someone could claim copyright on the image. But not purely procedurals. However I've seen .mat files with images that are otherwise freely distributable being restricted to non-commercial use only and I don't think this is enforcable.