The people that don't like contracts.
Mainly though they do that because of the people that have bad credit and can't get a contract and are able to get them both ways.
No Contract == Month to Month which is typically 20-30% more expensive
No Contract == non-subsidized Phone, which is big bucks at full retail.
For instance, my DroidX at full retail was $549. Month to Month "access fee" $10/month. Monthly average minutes/text/dataplan for a smartphone from Verizon is about $70/month plus surcharges and tax... etc.
AT&T is no better, Sprint just introduced one price month to month but I don't recall the price... dunno about the rest.
Except some carriers/specialist phone sellers like "Boost" caters to those that can't get contracts... they have plans starting at $40/month, period, including unlimited minutes/text/data... but man the phone sucks and its not a smart-phone. Get a *real* smartphone and the prices go up quite a bit. The cost is still based on your credit score, though... which means extra dollars for those not able to get a contract in the first place!