Prints are different I think.
Original paintings--I've seen tons of artists do this. It seems like a gimmick to me, but it is what it is. I don't think it's legal in a lot of states to tell you the truth. I doubt anyone is getting in any kind of trouble for it, but as far as I know, in California, you can't destroy an artist's work, even after it's sold to you (or the secondary market). It's still not "your" image to desecrate.