They do prevent you from linking to your own store within your Steam game though. Even though they don't provide a complete solution for things like microtransactions and DLC.
How it works on Steam:
User makes an in-app purchase using the steam wallet integration
Steam processes the payment taking 30% and gives you a reference number for that transaction
You query that transaction every time the player logs in to see if they've refunded it or not. That transaction doesn't actually contain any information about what they bought though.
You then maintain a separate purchasing server whose whole job it is is to keep a record of what the player purchased in reference to that transaction number.
For that Valve wants 30% of in-app/DLC purchases. At that point it's stripe and nothing more. Unlike standalone DLC Or expansions, these unlock purchases don't come with serving any additional content in the form of downloads.
If you make your own service to handle these transactions (with only a 3-4% transaction rate) Valve will prevent you from linking to it, or mentioning it anywhere on your page, forums or within the game itself. You need to direct players elsewhere and then mention it. Even for cross-platform games where having Steam maintain a transaction list for a portion of the users is just a needless additional layer.