I've always assumed that weak DRM is the result of a cost benifit analysis. A child with a hex editor could disable it in a few minutes, but most of your customers do not have a hex editor. And, of those that do, the ones who are willing to spend the time to break your DRM (however trivial) would probably not have bought your software anyway. The intersection of people who would work around your weak DRM, and who would pay if they could not do so is small enough that it is not worth paying developers to spend the time to implement a strong DRM.