I hope you tut at them loudly every time you walk past them.
I say again, what would the cost be to police the system so that person couldn't 'get away with it'? I believe it would be more than the savings you got by stopping them.
Ethical behaviour is not about not doing something for fear of being caught, it is about not doing something because it is wrong.
You are right - it would cost more to police that it saved, but that does not make stealing from your fellow citizens any less unethical.