if you can "modify" the server side and the client side, you can "modify" the need to pay.
edit: Sure, downvote me if you like. If you like to think adblock etc actually block all adverts I guess it's up to you. It blocks a small class of 'obvious adverts'. Several websites will pass outgoing links through a jump script which may or may not then go through affiliate links. Adblock is useless against such things. There are several such examples.
I don't do it to stop site owners 'making' money from me. If you can get non-annoying ads to me, and make money from me, that's brilliant. If that's all anyone did, I'd get rid of *block altogether.
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.