Correct me if I'm wrong, but something with a BSD license can be turned into proprietary software if you make a derivative of it. At least with GPL, all derivative software has to remain GPL compliant.
That's not really correct. The derivative work can be proprietary, but the original work is still BSD-licensed. So in no way has the original BSD-licensed code been "turned into" proprietary code.
And you could argue that being able to keep derivative works proprietary means that BSD-licensed code is more free (as in freedom) than GPL-licensed code. You're free to do whatever you want with BSD-licensed code, including making a derivative work and keeping that derivative work proprietary.
And it's worth mentioning that the GPL only applies until the copyright on the software expires. At that point, the code becomes public domain, and then anyone can do whatever they want with it, including making a derivative work and keeping it proprietary. So BSD-licensed code is a closer approximation to public domain code than is GPL-licensed code.
> And you could argue that being able to keep derivative works proprietary means that BSD-licensed code is more free (as in freedom) than GPL-licensed code
You are misunderstanding one point, though. Free Software is not about the creator's or the contributor's freedom, it's about keeping the piece of software Free to be modified. And a BSD license does not guarantee that the derivatives remain Free Software, hence that's why it's regarded as highly as GPL in that aspect.
True, and the MIT license can without even making a derivative of it (because you can pass on only a portion of the rights granted by the license and become a new licensor).
But the question is how much one is encumbering the work of others.
