It's quite common. Here are Richard Stallman's own words: "I've considered selling exceptions acceptable since the 1990s, and on occasion I've suggested it to

It's quite common. Here are Richard Stallman's own words: "I've considered selling exceptions acceptable since the 1990s, and on occasion I've suggested it to companies. Sometimes this approach has made it possible for important programs to become free software." and "I consider selling exceptions an acceptable thing for a company to do, and I will suggest it where appropriate as a way to get programs freed." and more. The whole article is worth a read: https://www.gnu.org/philosophy/selling-exceptions.en.html https://www.gnu.org/philosophy/selling-exceptions.en.html