I seem to recall a profile of the game and it's creators that said it's mostly by choice.
Found it: they were offered 300K just to license the name.
He has refused a programming job at a major developer
(he asked that I keep its name off the record) and
turned down a $300,000 offer from another company to
license the Dwarf Fortress name, fearing that the
proposed sum wouldn’t sufficiently offset the long-term
donations drop that would likely result.