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 cod

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.