It's not the viewer who is 'stealing,' it's the person who posted the picture.
The viewer (a child in my hypothetical example) is an innocent victim.
Consider what would happen if, to combat stealing, a store owner put poison in candy in such a way that he was sure that only stolen candy would ever be eaten with poison, and then the thieves gave this candy (with no reason to know that it was poison) to children for Halloween.
The store owner would certainly be liable if kids ate the candy (even though it was the thieves who did the stealing).
(I haven't conceded the point that hot-linking without permission is stealing)