Just because Picasso (or some living author if you prefer for the example) doesn't own one of his paintings anymore, it's not artistically acceptable (arguably even culturally acceptable) for the owner of the painting to go there and just paint something over it that totally changes the meaning of the original painting.
Just because you are legally entitled to do something, it doesn't mean you should do something. As a recent example, just look at the United Airlines example for instance.