This article made my day.
If you don't feel like reading it: This company sells a software that runs on Linux, Windows and MacOS X. It works on a key-based license, meaning you buy a "key" that you can use on any version. They noticed that
they don't spend as much time supporting people on linux as they do other platforms. So they decided to make a 25% discount if you buy the license from a linux computer.
They know it is easy to trick, as a matter of fact they say so on the announcement. I think it makes sense. It's their way of saying: "If you're a user that knows how to install a linux vm just for that, then you're probably costing us less to support anyway".
I think this may be the first time I hear of a customer-support story that made sense :)