Some people are just too afraid to do something illegal even if they, like you said, think they ar harmless. The reason I think crimes exist is because people aren't scared of the penalties. But there are some people who don't do it because of moral standards. If you can do it and not get caught then you will probably do it.
I think that the capital reason people don't commit what they consider "harmless crimes" is based entirely on the fact that they are illegal and the threat of penal charges. As anonymity and "safety" increase for the criminally intent the likelihood of the act goes up, hence why downloading illegally is so popular while shoplifting is so unpopular despite the object of desire often being the same media.