Dwarf Fortress devs themselves effectively defied capitalism and the game was simply donationware for the first 20 years of game development. Always respect when devs are able to do that and commit to it.
The official graphical versions now actually cost money on Steam, but they still release the underlying game for free (and plenty of free graphic mods out there if ASCII isn't modern-graphics enough for you).
I have a feeling they'll make the game open source when they die. Dwarf Fortress is their life's work, and they just want it to grow. If I recall they made the map generator open source.
i think they've mentioned in the past that the code will be donated and go open source after they've died. although i guess it would be torture to work on a 700,000 line 20 years old non-standard c/c++ codebase with almost no documentation with so many intricacies that touching one part will almost certainly introduce a bug in an unrelated place. i hope they get people onboard before they pass it on to the community.
Oh, the map generator is open source? I've always thought it would be rad to use as a jumping off point for building a D&D setting to play in, but using the game for that is impractical.