Are you saying that there is usually no government interference in private contracts? Or that there should be no government interference just when the consumers rights are being violated?
We're not talking about a legislated monopoly here, like your local power company. No one is FORCING you to go down to your local Verizon reseller and buy a contract phone. You're free to either buy an unlocked smartphone (at full un-subsidized price), or a prepaid phone, or any other of a multitude of options.
Once upon a time there was a group of people. They had common interests, but it was hard and inefficient to make decisions as a large group. So a government was formed to represent the individuals in the group and to tend to their common needs.
Let's go back to the present.
So one side is a big corporation that has one voice, the other side are consumers, i.e. people that have many voices. Government (at least a properly working one) is there to represent the will of the people and to give them a single voice. I don't see anything wrong with that.
P.S: I think it fair to suggest that what the majority of consumers want is the cheapest possible phone. That means locked phones on subsidies.
Regarding your second point, the assumption is that the government is rational. Adult-like and not child-like.