The origin of the face towel

I guess most social conventions are invisible to longtime residents. As a newby, I'm puzzled by this - a dainty little towel that cloaks the meter in all the taxis I've travelled in so far. Is this a gesture of discretion, so as not to tarnish the interaction between driver and passenger with the grubbiness of commerce? Without the meter, the journey might be characterised as a mate giving a mate a lift. It's a nice thought. Less generously, in masking the meter the driver might hope the passenger either doesn't notice there is a meter in the cab, or is content to let the driver charge whatever his imagination suggests a journey is worth? Whenever I've asked to see the meter, the neatly arranged towel has been lifted only grudgingly, only partially and only very briefly as if something deeply illicit hid underneath. The truth is a dirty secret! Anyway the towel is clearly a deeply embedded social convention, at least in this city. Somewhere, sometime, the first driver threw his face towel over his meter and a craze was begun.