What is their fault is using their monopoly status to charge 30% of sales for an online storefront. For many games, Steam's cut is the single largest expense.
If you're a developer of a game being sold on Steam, Gabe Newell's personal cut on the game that wasn't produced, published, or marketed by him or any company he owns is more than yours because he charges an unconscionable toll for the storefront.
If Steam charged 5% instead of 30% they'd still be making a killing, but since they have an an effective monopoly it doesn't matter.