Had to look it up for curiosity's sake.

In order to get fully set up, you will need to pay a $100.00 fee for each product you wish to distribute on Steam (the "Steam Direct Fee").... This fee is not refundable, but will be recoupable in the payment made after your product has at least $1,000.00 Adjusted Gross Revenue for Steam Store and in-app purchases. Steamworks Partner Program

While not free, seems to be no overhead for the length of stay on Steam that I can see. Another site includes some more description which might add to the cost though

The cost to put a game on Steam depends on the type of game, the development budget [1], and the business model. To get a game onto Steam, developers need to pay a one-time registration fee of $100. After that, they will need to pay a 30% royalty fee. Additionally, developers may need to pay for marketing and other costs associated with the release of their game. source

Regardless, it's just another unfortunate case of physical copies/backups>subscriptions or "licensing/rent" deals.