@jimnms: You missed the point that account is required:
To make use of the Software, you must have a Steam Account
That's not a may be. So if Steam closes your account for whatever reason, or let's say Steam itself shuts down (and naturally your account is gone), you won't be able to legally play your Steam games anymore. They can't really enforce such restriction but as I said, practically all DRM is unenforceable and as such is completely nonsensical to begin with.
Can't say anything about games on Amazon - I never looked into that service, but their other services are DRMed (e-books and video for example). The only DRM-free option that I saw there were music purchases (where you can download the files).
