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They can and had done it before, see the link at the bottom of the article. Basically, game devs are forced to sell the game at the same retail price in all platforms regardless of the commission cut of the platform according to Steam license. BUT as a customer, usually other platforms are more expensive, so mileage may vary. I like Steam a lot and support it whenever I can but if there is evidence of wrongdoing I would change my mind, however, the complaint from the article smells strongly to cashgrab.
Do they? As a long time user of /r/gamedeals and isthereanydeals that is focused on game sales I’ve got tons of games cheaper than they were being sold directly through steam. Humble monthly being one of the best with sometimes price of like 8 game bundles being less than the cost of the cheapest price a single game went on sale.
See the comment from furikuri