Originally Posted by borked

Not at all entirely true. It depends what license the OSS is bundled under. The BSD license grants open source or proprietary use of its software (classic eg is Apple's OS X operating system), whereas Gnu Public License (GPL) requires any derivative works (which can be saleable) to be distributed in source code under GPL or compatible license. People get around the GPL license by daemonising their software, in order to avoid creating a derivatinve work.