Am not sure in the US but we certainly have a problem with unlimited data plans as they bring in channel stickiness [still in 2.5G] and tend to screw up capacity planning on the Radio access network. Remember telcos have to meet certain SLOs set by the regulator and capacity/coverage planning play a major role in that. Revisiting these phases would add a substantial cost onto the network.

So telcos would prefer to control the TCO
a) by offering fixed usage models and
b) pricing very high on the exceeding bytes to offset users from the network.
c) Not allowing Packet calls during Peak periods