OK, I've talked with some of my friends and found out how they filter the "dud" stuff from programs like kazaa vs the real stuff...
Basically when you do a search for Kazaa, you can find out how many people has the same stuff (or more correctly, I guess it refers to how many instances are available at the time of your search). The chances are, if the program, game, music, movie etc you're looking is really popular then there's going to be a lot of people who have it - which means if you download something which happens to be on a lot of computer's out there, then it's the stuff you're looking for.
So while the developer could put on some computers out there a dud copy, are they going to be putting in on enough computers to make a difference when people are using the above method to choose which copy of the <something> they want to download??
Keep in mind most broadband contracts either charge by the megabytes, or else (if it's on an unlimited plan) don't usually allow the user to set the computer as a server for people to download from... so putting enough dud copies out there for people to download from is going to cost a lot of money, with no gurantees (if the pirate is persistent enough, then he/she can always seek out other sources).