https://www.greasespot.net/p/privacy-policy.html
It seems fairly standard.
We may collect non-personal identification information about Users whenever they interact with our Application. Non-personal identification information may include the list and/or number of installed scripts, the source they were obtained from, metrics describing usage of the Application and User behavior and Telemetry, and other similar information.
We do not sell, trade, or rent Users personal information to others. We may share generic aggregated demographic information not linked to any personal information regarding visitors and users with our business partners, trusted affiliates and advertisers for the purposes outlined above. We may publish aggregated statistics based on this information online.
So basically they can "share" bulk user data with others, but don't sell or trade it. What Stylish started doing is a lot different and people were very unhappy about it.
That sucks. Please tell me Grease Monkey isn't doing the same.