The thing is that the $15 minimum would have applied to all of the services that Uber offers, not just their town cars. This would have an impact on the rollout of UberX to the area, which has a minimum fare of $10.
Exactly. This was an attack on the new "Uber X" service which will more directly compete with regular cabs. (Frankly the only reason I even take cabs anymore is the cost of the regular Uber service.)
http://blog.uber.com/2012/07/03/sf-vehicle-choice/