Steam itself doesn't force DRM on a developer. As an SDK, it provides a very easy DRM platform to choose to use. They offer it, but it doesn't have to be utilized.
Currently everything needed is reverse enginered so you can actually download games using open source tool, but I pretty sure this is forbidden by Steam EULA. Even if we all know Valve will unlikely enforce it it's still gray area you don't want depend on.
The fact that you need a client to download your game is not DRM, but it is inconvenient I suppose. DRM comes in when you cannot distribute your purchased game freely, and you're limited to a set number of PCs or installs.
Still disagree? Let's look at GoG, the standard DRM-free game store. You need to login before you can download your games, correct? Would you call that a form of DRM? The difference between logging in and downloading a client is negligible in this day and age imo.
If you don't agree with any of the above, then I'm afraid that the way you think of DRM-free gaming is only achievable through piracy.
I personally find Steam, buggy, ugly, intrusive and effectively spyware that is logging and tracking god knows what. I feel very uncomfortable installing it on my machines and in general having to go through software A to get to software B makes a lot of people uncomfortable.
By contrast, logging in confines the the website to tracking my activity on the website itself - which they probably can do with a cookie anyways...
The only difference between Steam and GoG is that GoG runs software A on their servers and in your browser, whereas Steam runs software A on your machine in the custom-skinned browser they give you.
Any security or privacy ramifications of the two different approaches are quit irrelevant to a discussion of whether it constitutes DRM in the generally accepted sense of technical measures to prevent you from copying, sharing, etc. digital content that you already have in your possession. It's not DRM to require some authentication before they'll serve up the download to you. DRM is what gets in the way after the download.
You can have open source DRM (for example hardware enforced schemes).