Rossmann and Futo have explicitly stated that purchasing their soft is a purchase, not a donation.
Exactly. That’s the issue. Selling software means generating revenue, this Grayjay is a commercial product. That’s exactly what YouTube Vanced was shut down for. Generating revenue from accessing YouTube in an unauthorized manner. Grayjay does use the YouTube API, but not the official one. YouTube has 2 sets of APIs. The official one with clear terms and conditions, which is only accessible with an API token, and the unofficial one, which is for example used in the YouTube mobile app. There is no official documentation on this API, and it was only discovered through reverse engineering. YouTube doesn’t allow using this API from an unofficial client (they’re not that strict about it, but nonetheless, it’s technically forbidden, as the APi is not meant for public usage).