[topspin]

Typically the fob is the transponder. The fob has a tiny battery: frequent transmissions would rapidly kill that battery. The car has an ample battery, so it transmits periodically and detects the responses from the fob transponder.

For the purposes of a "relay attack" this doesn't actually matter: all else being equal you could devise a relay system that works regardless of the roles of car and fob in the protocol.