Sorry leafo, I didn't have too much time to reply soon.
It seems like these days itch.io player already knew this issue often.
https://itch.io/t/169922/is-it-appropriate-to-sell-just-a-steam-key-with-no-down...
And you are right and wrong.
First, you should correctly name they are distributor not publisher.
These developers have their own publisher (usually themselves) and they corporate with these distributors Plug in Digital and 4EversGames let them set the market.
There are not many distributors on itch.io indeed, only two as I saw, but they are too often making sale, and care nothing about the store page and further support (such as update news, dlc and main game that are only published on Steam for people)
In this 2017 holiday sales, it is the first time I feel that these distributors set the sale for long time already, and put so many sale titles (almost 16, and no different introduction and details), the most important problem is that their huge amounts are reducing and threatening in every section visibly.
That means when the players want to see some indie game made by hardworking individual developers, will have to pass through these big company' stuffs.
I have discussed this with Mini Metro which is one of the game distributed in itch.io, and the publisher told me that they will soon cancel the corporation and do it by themselves.
Itch.io should be careful on this issue, at least there are many possible way to try, such as creating a Distributor section for every distributor, and for those who only sell their products as only Steam key, that is something conflicting itch.io main idea.
I just found your word in an interview with Rock, Paper, Shotgun, which is the itch.io original motive that seems be breaking now:
"It gives you the ability to design a custom page where people can download, play or purchase your games."
https://www.rockpapershotgun.com/2014/04/23/steam-itch-io-digital-retail-intervi...