You aren't paying for 4G service. You are paying for them to expand their network. It's spelled out in the contract. They changed this a few years ago when they started charging an extra $10 for the 4G phones but was unavailable to anyone. So they changed it to a premium service fee, which they can do.
As far as poor reception where you live, call them and tell them. They should send you a device which triangulates towers and serves as a mini cell tower for your location.