I totally agree. Imagine you've got a bad tummy, you feel knackered and ill, and people are approaching you wanting conversation about your job? You wouldn't appreciate it would you? I think people and their personal space should be respected and left to their own devices unless in an official work capacity.
It's a fine line. On the whole I don't think people not obviously busy (eg in the middle of a famiy meal) mind if someone comes up, pleasantly tells them that they love their work, and passes on. Well I wouldn't mind. I would never approach a celeb whose work I didn't like - what would be the point?