because MC doesn't know the difference between usb devices that are explicitly used for storing data or music. Since data could be music.. i.e. mp3 files, you could theoretically use your "usb data thingy" (I have one too.. I know what you are talking about) to transfer new music to it all of the time. You could even store lower bitrate versions of your music to cram more stuff on it.
Also, some usb flash drives can be converted to an mp3 player if you get an additional attachment. It depends on the model.
My computer still thinks my camera is a hard drive when I use my usb trasfer cable so sometimes MC tries to make that a synchable usb device too.
