1) You can tell them that it is THEIR art. They saw it first. They have first crack at it.
If we were out hunting and you were taking aim at a big deer, would it be right if I shot it out from under you and dragged it off?
2) It might be instructive to hand somebody a camera and let them (try to) use it. They might learn that this game ain't so easy as it seems.
3) Who knows? Maybe they'll get the bug?
4) Even if the picture doesn't turn out great, maybe they could use it as a reference for when they decide to make a painting, a sculpture or whatever. (See #1 above.)
Yes, I understand that the people were only trying to be nice but, like you said, you were busy. I don't think that it would have been out of line if you said it nicely enough.