While I disagree with your example (I would *never* give a GOG installer or a Humble DRM-free installer), I got your point… and still disagree with it ;)
DRM-free installers can’t be restricted, by the very definition of DRM, but the access to them could easily be. It is based on trust, and there is nothing that can be done about it.
On the other hand, Humble could easily disable the access to the DRM-free installers if you give away the associated Steam key, *without* letting go of the DRM-free principle in any way.
Just to be sure that we’re on the same page: I’m not saying Humble is encouraging you to give away you Steam keys while keeping the DRM-free installers. But they are not in any way discouraging you to do it, while it would have been super easy to put a restriction in place.
In my opinion they trust us to use this system without abusing it. Buying a whole bundle for 1$ to then trade the Steam keys and keep the DRM-free installers is abuse in my book, while buying the same bundle for 10~20$, give away a couple Steam keys to friends and keepind the DRM-free installers for yourself sounds like a respectful use of the possibilities Humble give us.
Of course, feel free to disagree, but keep in mind that you will have a hard time convincing me my ethics are broken ;)
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Just to be clear: I do not own, and will never own, a Steam account. So I have no use of the Steam keys for myself.