Valve's strategy is actually the norm for the business world in general. The more business you do with a supplier/platform, the cheaper your unit costs get. If you're a small business buying 100 widgets, the price per widget is a lot higher than if you're a big business buying 100,000. Same with normal SaaS - try calling Stripe and asking them for a fee discount on your $100 revenue/month app vs. asking them for the same discount on your $1,000,000 revenue/month app.
The reason Apple doesn't use this normal model is that the mobile app market is completely screwed up - you get exactly two real choices for distribution if you want to make any money at all as a small developer.