gammaleak: Hmmm... this seems like an attractive explanation... but they have released on other distribution platforms.
No, they haven't, at least NOT on PC. Steam is the only digital platform to sell old id shooters. Well, unless a smaller company i'm not aware of has them too, but i have never seen these games on Impulse, Getgames, GMG, Gamersgate, Amazon, Dotemu, Nuuvem, Gamefly or anything else.
However, Doom is available on Apple's App Store (for iOS devices) and XBLA, but these are not PC distrbution platforms. The XBLA version is actually a revamped HD version with 4-player coop support. This HD version is not available on PC.
EDIT: Looks like Impulse has a few id shooters too, but i have never seen them because of nasty region locks.
jungletoad: I'm pretty sure that the old iD games are some of the few DRM free games on Steam. Copy them out of your steamapps folder and I think you can play them on other machines or without Steam.
Probably, since these games use DOS-box and they can't force their DRM into DOS mode. Just like X-com games.