Why? It looks like an open contract for me, because price is an essential part of the contract. A contract with 0 price for all future labor is just slavery. Slavery is forbidden by law. Thus, you can make a game and then ask for reasonable compensation for your labor, e.g. $1000 per line of code.
Since you're not forced to contribute the changes back to their repo, or even tell anyone that you made any changes, the slavery metaphor makes no sense to me.
