I'm a gamer and (amateur) gaming dev hobbyist. These distributors both trouble and befuddle me a bit. I don't feel that Valve would be ok with anyone else flogging Steam keys on other platforms to bypass the Steam store. That would be a violation of Steam's Developer Agreement. It feels as if devs are doing these 'distributor agreements' to pay less than Valve's 30% cut while maintaining all the perks of the Steam marketplace.
I don't feel that itch.io should permit key-only selling by devs, publishers, or distributors. If they want to only sell Steam keys they can do it on Steam. If they don't want to do it on Steam then they can do it on Humble or IndieGala as well. Itch.io isn't really meant to be a Steam key purchasing store, nor is it set up properly to that end.
In a worst case scenario, Valve themselves could get involved and start terminating publishing arrangements on Steam with those that violate their Steam Developer Agreements.