why is it free?
lots and lots of testers who do the work for free.
we basically find bugs and provide feedback at no charge
A lot of games of about this caliber are free. There are, at this point, more muds and mud codebases than you can shake a stick at, mostly free.
Multi-user dungeon games are afaik made by teams and are made all at once and possibly never updated, the number of inactive/deleted MUD huds on a program such a fire-client far outnumber the serviceable onces.
Anyways my point was Toady could probably work as a programmer or even software engineer (don't know his background) in a company and get paid big bucks. Instead, he does what he likes to do (I would think) and relies on donations, which I admire and respect.
P.S. If Tarn Adam's sole source of income is Dwarf Fortress than it is a no brainer why he would keep the DF source code secret.