No 'slippery slope' situation exists. It would be clear that the ban only applies to music that glorifies violence. This is not a justification that could be infinitely expanded to cover more and more music and art. It could not be considered a precedent to ban music with a political message as most political messages do not promote violence.  Far from stifling creativity it is likely to stimulate it. Artists would need to find new styles of music and would attempt to find ways around the ban while still keeping their music as near to its previous style as possible.