I suspect that it's mostly an issue of money. Releasing a game, even for free download, costs money in the form of staff, servers, bandwidth, etc.
In fact, Commander Keen, a game that was released by iD Software 20 years ago, is available on steam right now for $5.
Naturally, there should be some drop off point in sales where the PR becomes more worthwhile than the couple bucks.
EDIT: MechCommander was released as a torrent. Microsoft (you know, that Microsoft) owns the IP.
A game via torrent? It's probably already been "released" for them...