You are genuinely deceived.
You forget that Apple also signed the contract. It was a two way street. And the contract has a specified term, ie its not permanent.
Apple will be free to go with anyone should they choose to or, OH MY, continue with their current carrier.
Yes, and me signing myself into slavery would be a two way street.
You seem to have missed the essential point. Hint: it isn't about Apple. It's about the entity who is currently one-half of a two-party oligopoly which effectively controls a vital market. Two parties which, in fact, were only relatively recently split apart after having a monopoly for a century.