> I often receive emails from people who ask me to release my software under MIT or BSD3 rather than GPL so they can "relicense" it,
That makes no sense. People are asking GPL software to be relicensed under BSD/MIT to make them usable in situations where the GPL is a legal hassle or impossible to use. I doubt that people even consider relicensing BSD as something else (besides people practically downgrading BSD to GPL).
1. User wants to use my GPL'd library in their application. Their application is released as source code under the 3-clause BSD license.
2. User sends me an email asking me to change my library's license to MIT or 3-clause BSD so their open-source app can use it.
3. I reply with the longform equivalent of "u wot m8?"
4. They reply that if I change my license to MIT, they will be able to "relicense" it to 3-clause BSD and use it as a dependency in their application's build script.
5. I explain that they don't need permission to just depend on a library, since they're not distributing any of it. And besides, if they did include some of my code in their tarballs, they still can't change the license because they don't own the copyright.
I suspect this stems from the same sort of pseudo-religious copyright knowledge that leads to "no copyright intended" disclaimers on YouTube. Lots of people grew up in the era of Napster and copy-pasted MySpace javascript, and never really gave much thought to how copyright actually works.
Presumably, User wants to include your library in their open-source BSD release. If that is the case, what they're asking is correct since a work extending / modifying a GPL licensed code needs to be released as GPL as well. That is the main difference between GPL and non-copyleft licenses.
They could distribute their code without your library, as you say, but that would require more hassle from the end-user, especially if there's some configuration / integration process that is required to make both work together.
My libraries are mostly in Haskell, so depending on them is a matter of adding a line to a standardized build file. I have not yet received mail from a user asking me to change a license because they wanted to actually include code in their works -- it's almost always a misunderstanding of copyright.
Presumably what they want to do is mix the BSD code with their own code and release the whole under a restrictive licence. That's permissible under the BSD licence, but doesn't involve relicencing the BSD code, since that's still available under the BSD licence.
That makes no sense. People are asking GPL software to be relicensed under BSD/MIT to make them usable in situations where the GPL is a legal hassle or impossible to use. I doubt that people even consider relicensing BSD as something else (besides people practically downgrading BSD to GPL).