It's been closed source for 16+ years. Some background:
* https://blog.paint.net/2007/12/04/freeware-authors-beware-of...
* https://blog.paint.net/2009/11/06/a-new-license-for-paintnet...
> The biggest problem was that, even though these were clearly unethical and deplorable actions, the MIT License permitted all of it.
Seriously? Why not go GPL then?
> The biggest problem was that, even though these were clearly unethical and deplorable actions, the MIT License permitted all of it.
Seriously? Why not go GPL then?