"Steam keys are meant to be a convenient tool for game developers to sell their game on other stores and at retail. Steam keys are free and can be activated by customers on Steam to grant a license to a product."
They didn’t have to acquire the user themselves and now they have more eyeballs on their service. How many of these users buy a few things later and make waiving the fee worth it?
Steams not doing this out of the goodness of their hearts.