Indeed we can't do anything about.
And yes, always glad people are buying.
I'm my case i was wondering it cause a buyer of my latest mailed me with questions and was telling me he or she was working on two sites with it.
Besides that, they also asked where the source for a theme module was, so they could use it in another project.
Well, i did mail them back that such thing is realy out of the question and pointed him to the license page and said he could use it once. But i also said i'm not going to make a big deal out of it since there's nothing i could do realy..
My guess is there aren't many 'good guys' as BenSheppard describes. And also lots of theme functions get moved out from themes to other projects.
Allthough it's too bad for authors, maybe this lack of control is what makes buyers buy great items and there are a lot of sales...