Thing is, from the law's point of view, games that you get in a box, or a DRM-free download from GOG or whatever, are still only licensed to you and require you to conform to the requirements of the license.
The thing that Steam can do, which traditionally boxed or DRM-free software cannot, is to actively RESTRICT access to the software you purchased, on account of the authentication features built into the client. Put another way - if I have a game in a box that doesn't need Steam, so long as I take care of the media (particularly if I make a backup copy, or create an ISO or whatever), it can last for however long I want to because I'm the one in control over the software. Steam remove the user's control and places it squarely in hands of Valve and the publishers. If the Steam client fucks up, or your net connection goes down and Offline mode decides to not work, or a publisher/Valve changes the terms of service/agreement to an unfavorable state, you might find yourself unable to play the games you bought.
Boxed/DRM-free software and Steam software both share the same result - a license to use the software. But Steam is the only one which can actively police things on that front. Unfortunately it also makes things vulnerable to actions that aren't the user's fault.