I don't think your position would be very compelling in court. By analogy you could argue that adding a movie to your hard drive is 'commenting on it' because its inclusion represents an implicit comment of liking it. And the courts have already ruled on that. Basically I find it hard to believe that you will get away with copying something by arguing you like it and that's a comment. If you could anyone could copy anything.
I disagree, I think the situation you're talking about sounds like the classical napster/kazaa/bitorrent kid with a ton of songs on their hard drive who is ripe for legal action, whereas the pinterest situation -- even in a worst-case martial-law scenario -- is more like you've streamed a movie to your hard drive from Youku or something like that.
The end user, when they don't have anything sitting in their house, is much less liable to be prosecuted.
