When you buy a game from Jason Rohrer, you get the source code too. (It's also on GitHub.) What you're actually buying is an account on the official multiplayer server (which is also open source).

I thought this was a really interesting model.

Maybe it wouldn't work so well with singleplayer games. However, there's a similar example with the sprite editor / animation software Aseprite, which is open source but you have to compile it yourself, so it's cheaper for most people (in terms of time and effort) to just buy it for $15.