Snippets of BSD code have ended up in all kinds of GPLed projects. It would be kind of "rude" to just take a BSD-licensed project in its entirety and re-license it. It's legally questionable too. As someone who isn't the copyright holder, you can't really dictate the license.
In fact, incidentally, I believe that if you integrate BSD-licensed code into a proprietary product (where, say, it is manifested as a translation to machine code), you don't actually own those sections of machine code. They are derived works, governed by the BSD license: anyone can rip that out and use it accordingly. The code has not been turned proprietary at all---it just appears in a form that is difficult to modify and tightly coupled to proprietary code.
