It isn't their rights. They are dead. It's a very simple concept. The rights belong to whoever gets the property. Also intellectual property rights isn't about respecting artists. It's about protecting corporate rights since most intellectual property is owned by mega-corporations and not the artists who created them.
> but we don't respect the artist's desires about what to do with that property.
Who is 'we'? The only person who has a say in the matter is whoever gets the property after the artist's death.
> but none of it is a natural law: it's a set of somewhat arbitrary legal decisions that we made as a society.
I know. That's my point. But regardless, it doesn't change the fact that the artist is dead and has no rights.
Should the sons have respected marquez's wish? Maybe. But then again, the sons probably knew their father better than you did. And marquez must have left the works to his sons for a reason.