It doesn't. It is an open source project. Originally Sandstorm was offered by a startup, but they stopped [0] in 2017. You can make donations though [1].
[0] https://sandstorm.io/news/2017-02-06-sandstorm-returning-to-...
[1] https://sandstorm.io/news/2021-06-17-sandstorm-community-fun...