I think far more programs (even those compiled into machine-code) are simpy copied and used without due respect to the laws of copyright, than programs are decompiled, studied and incorporated into other code.

Do not over-rate the intellectual capacity of the users of your programs (any dumb-ass with a CD-burner can faster copy and distribute your program than you can write it), nor deceive yourself about the quality of your programming: it may be a shock to all of us programmers, but the world at large is most probably not interested at all in how we programmed things, just that it works (and how they can get away with it without paying).