Have you ever bought a game via a Steam key? Those use Steam's infrastructure for distribution while not providing them (Steam) any income.

As someone that has entered so many Steam keys over the years (over 1,000) yet only bought maybe 20 games from Steam I almost feel bad, but the dev was following the rules and Steam is okay with it (as long as they aren't selling those keys for less than they can be purchased on Steam for).

Amazon provides ad and marketing services, hosting, support, and more importantly, logistics.

Yes, but if its not sold on Amazon, they don't do these logistics.

Games sold outside of Steam have no access to Steam features

And steam allows that too. Their only problem is with distributing the steam keys yourself for less.

The act of reading is something only the fewest people can do, apparently