From what I've seen in the case of CoD, Steam is simply a retail outlet. They do require an account to make the purchase for some reason (I wonder why that is - lol). Since they aren't tasked with DRM for CoD, for those who purchase elsewhere a steam account will only be required for use of Steam services. Those who wish to forgo these services may do so effortlessly.

This is just my understanding from the official posts. (define: official ... )