An aircraft's transponder doesn't send transmit the same code all time. The air traffic controller taking charge of a plane's movements gives the pilot a 4-digit squawk code to enter into it. That code ceases to have any relevance when the plane leaves that air traffic control area (e.g. when it's over the middle of an ocean) so it's a perfectly logical step to switch the transponder off at that time (to avoid transmitting an incorrect code when entering a new ATC area later on).
The only time a pilot might then turn the transponder back on would be if he declared an emergency and sent out the emergency code (7700). Even so, if he was out of radar range of an ATC centre, there would seem to be little point in doing so.