The end-users can do whatever the hell they want. Including pirating it. If they do anything wrong, they'll be the ones answering for it.
What was provided is a benefit to them. How they use it is to their discretion.
Hold it, I missed this part of the post.
So, you say that if a game is available in Windows, Mac and Linux versions, you can keep the Windows part for you, send me the Mac version, and trade the Linux version to Amok, thus essentially getting 3 copies from your 1 license?
The distributors that provide steam keys in addition to the DRM-free downloads are not giving you an extra copy, they are allowing you to choose the platform on which you wish to play your game. So yes, if you keep the drm-free installer for yourself and pass the steam key to someone else, it is the same as if you just downloaded a copy for yourself. Only time I'll accept it as acceptable is if you already have the steam (or gog, or desura) copy of the game, thus you could argue that you are passing the previous license to someone else, which may or may not be ethical.
Same thing as if I buy TW2 on steam (or retail) and give the GOG redeem code to someone else, without already having the game on GOG.
Do you still not see what the problem is, and still go with "Users can do what they want with their one license"?