But what stops them from reslling it or extending it themselves?

The answer is: The only thing that *really* stops anyone. The license/contract the developer gave to the customer. If the customer violates the license, you sue them. It's apparently the American way. :)

Just because something is binary, packed in a jar, etc. does not mean it is impossible to "resell" or "extend". It just makes the extenstion part more difficult, not impossible.