Well, technically, Steam is a by-subscription game-on-demand service. Its not really the same thign as DRM, but the end results are the same.
One of the goals for Steam was to allow developers to create a game and then sell directly to the consumer and skip the whole publishing phase. If there's one thing people around here on HardLight know, its that publishers area great source of evil in the game industry. From Interplay to Infogrammes, dumb decisions are made by publishers constantly and developers generally get the blame.
The problem, of course, is that Valve forgot the end user in the design for Steam. Sure, its great that your game is always up to date and always has new content without having to install mods or addons, but what if you can't get to the Steam server that has your game? Suddenly, there's this software--that you've paid for--that you cannot run, or play.