The biggest reason that Apple really does not allow for installing IPAs on their devices is not really because of the money issue (though it is part of it), but it's also in part that it can be abused to install unauthorized packages, and could eventually lead to exploits. For the jailbreaking community, that would be the best thing to ever happen. For security purposes, it's a huge flaw that can open your device to others that have malicious purposes.
In a theoretical sense, say that you can install apps on your own, and you download something off the web, and you install it. You won't know what the uploader has done to the package. They could've injected spyware in there, etc, etc. Pirates really only think about the free part of their app and nothing else. Aside from the legality, security is pretty far up the list.