You are partly mistaken on this. it is true some of the cost of the subsidy is built into your monthly bill, but the US carriers took huge hits to their bottom line paying iPhone subsidies over and above what they recovered in subscriber fees, particularly in the early days of the iPhone. During the early days when AT&T was the sole iPhone provider, they got just killed by this. There are some articles
here and
here. There were even reports of carriers secretly encouraging employees to steer customers to Android phones so they could avoid paying Apple's higher subsidy rates (article
here).