You’d have to ask the developer, but it seems like they did not understand the license they released it under. They complained of people “plagiarizing” their source code after releasing it under the MIT license, which allows people to take the source and use it however they want.
You can release your source code without making it freely available for people to use. Not every open source app is GPL licensed.
then why did the Paint.NET developer go from Open to closed?
You’d have to ask the developer, but it seems like they did not understand the license they released it under. They complained of people “plagiarizing” their source code after releasing it under the MIT license, which allows people to take the source and use it however they want.