I agree with pinotoon! You should leave this option open for those who want to take the risk! It means to me because of the presentation of the models , but the risk certainly exists ... also models from any 3d engine can be ripped! Private game engine, cry/unreal/unity... webgl, etc.
Again, some precautions are there, of course, on the sketchfab forum they certainly understood it seriously . Also I think models are inside special archives .gz files that doesn't seem to open on first try...
As someone from the sketchfab forum said:
" Your original files are never exposed unless you enable download. Our viewer serves a compressed version of the mesh. However, once the 3D data reaches your GPU, it is available locally. This is required for our viewer to work. This is really no different than any other media published on the Internet; it's trivial to rip images, audio, and video from most sources. In fact, ripping 3D is harder.
That said, we are working on some features to make such techniques harder. In theory, it's possible to make automatic ripping VERY difficult. However, this would hurt performance and stability of the product "