Switching our carrier is allowed under FCC's guidance. However, the carriers like ATT, Verizon, Tmobile hinders by only allowing to change service on a particular day (like end of monthly billing cycle) or else require to pay an entire month fee.
T-Mobile's policy : http://www.t-mobile.com/Templates/Popup.aspx?PAsset=Ftr_Ftr_TermsAndCond... (see condition # 3)
Verizon's policy:
http://www.verizonwireless.com/b2c/support/customer-agreement
(see section # CAN I TAKE MY WIRELESS PHONE NUMBER TO ANOTHER CARRIER?)
This is unlawful, and subjective to carrier's advantage.