There's some history there. Basically a scammer was passing it off as a paid app [1].
See also [2]:
"Excluding the installer, text, and graphics, Paint.NET was released under a modified version of the MIT License.[6] It was initially released as completely open source, but due to breaches of license, all resource files (such as interface text and icons) were released under a Creative Commons license forbidding modification, and the installer was made closed-source.[7]
Version 3.36 was initially released as partial open source, but the sources were later removed by Brewster, citing problems with plagiarism. In version 3.5, the license was altered to reflect this, and users are now prohibited from modifying the software. As free licenses cannot be revoked, developers can still legally develop forks based on version 3.36 and earlier. Brewster later stated that he hopes to release portions of the source code back into the public."
If the author see no obvious benefits with keeping it open source but sees downsides I don't see why it would be an over reaction. I did exactly the sane thing as he a few years ago. I've since opened sourced it again since people were wanting to contribute.
