Because laws are there to fix asymmetries in negotiation power between parties. As you can imagine, artists and art collectors/galleries are not really on the same level.
However, there's still the question of why this particular thing should have that protection. You could well stretch this and say engineers with massive student loans are equally up against the wall, and should get paid a piece if the code they write turns out to be very profitable later on. They don't all get equity in the company.