In a sense it's worse than that - when the mega-publishers started leaving Steam in favor of their own stores with better margins, Valve conceded by introducing a tiered system where their cut drops to 25% after $10M in sales and again to 20% after $50M in sales. It's deliberately structured to give a discount to the mega-corps which least need it (EA, Ubisoft, etc) while ensuring that indies with no leverage over Valve mostly remain on the 30% bracket.