Steam is DRM. Can't play without having authenticated. It infects everything or damn near everything these days. Steam is trying very hard to get a monopoly. They want to lock out competition. They want to control everything you may do with the game you bought (rented might be a better verb).
Because of all that what more reason is needed to not buy a particular game?
It's up to the publisher, you can have a completely DRM free game on Steam. For instance, I just copied the FTL game recently from my Steam install folder to my netbook to play on a trip. Steam has never been installed on it.
Indeed, but it is true that Steam has no indication whatsoever when you download a game (or buy one) whether it's DRM-free or not. It's not explicit in any way.
