The whole updating/upgrading argument can be made for all software. Many pirated versions of operating systems and major software suites can't be updated, but this doesn't stop a huge number of people from pirating it (Windows, Office, and Photoshop to name a few).

There are plenty of apps on the app store where all you have to do is zip up the application's directory and send it to a friend. This means that you don't even need to log into your account on their Mac. All you have to do is install the update, zip it, and toss it in dropbox or send it in an e-mail.

This is a gaping hole in Apple's infrastructure. While circumventing piracy completely is impossible, it really shouldn't be this simple either.