Their implementation is copyrighted. What prevents royalty-free clones is patents. MIPS was similar with certain key patents preventing clones until their recent pivot. You can clone legacy ARM architectures that aren't patent encumbered.
I think most posters in this thread are forgetting this point. RISV-V will have a key advantage because not only will it be free, but you’ll have access to high quality free implementations you can redistribute and remix without copyright issues.
People seem to forget that manufacturing and distribution of hardware is the expensive bit. "You" is never going to be an ordinary member of the public.
