Huh? They don't have to tell you they're using Linux either. That's why people make noise every now and then when a company is discovered to be using Linux in its embedded product without providing the source as required by the license. Not every case like this is going to be noticed by anybody, because you have to have some insight into the system (such as recognizing the mplayer strings in the binary, or a TCP fingerprint, or something) in order to even devise a theory as to the OS it's running (Linux, BSD, whatever).