These are the same teenagers/college-age persons who own $600+ iPhones.
(for "subsidy" to be a reasonable description of the flow of funds, the phone company would probably have to be losing money on the most expensive phones; of course they aren't. They provide the phone up front with a minimal payment, but they have structured the monthly fees to more than cover their costs for doing this.)
Furthermore, early termination fees are constant for all contracts. The person who got the $150 phone has the same termination fee as the person who got the $600 phone. Thus again, someone is subsidizing someone else.
And the fees aren't necessarily the same for cheaper phones: