[dave_au]

If you can arbitrarily modify the server side of any instance of the application then you can also just pay using someone elses credit card details.

Although I think what Stallman would probably prefer is the source to the site / web application being available so that people can run their own instances.

HN and reddit have their source available. I don't see anyone using the reddit code in a way that's any threat to reddit, mainly since the size of the community is an asset that's hard to compete with.

Potentially off-topic, since I'm not familiar with the prevalent licensing, but the various django applications look pretty nice for this kind of thing.