The 2% is stealing because no reasonable person would expect to see such a clause in an open source software project. The 2% clause was hidden, all the way at the end of the doc. The plugin author is a conman.
Why do you call it Open Source? The plugin itself does not call itself Open Source, and clearly links to another project for those people who want an Open Source program.
The code being free does not mean usage is free. Qt’s code is free to look at, but you still need to pay them for certain things. Licenses apply to the code, not what it does.
This is used for popular game engines. It's not a bad model. If you make nothing you pay nothing if you profit you pay a %. It removes the need to buy upfront.