... Apple is silly not to sell on other carriers ...
It's not Apple's decision. Even if their AT&T exclusivity were expired, Apple couldn't just start selling a device on e.g. Verizon.
Consumers are trained to expect several hundred dollars worth of subsidies when "buying" a phone. The carriers control the subsidy, so they decide what devices go on their network. (Initially in 2007 Apple tried the waters with an unsubsidised $600 iPhone. That didn't work then, and competition is much more fierce now.)
The surge in Android marketshare is strong evidence that Verizon can do very well for themselves without Apple. I'm not sure if Apple's bargaining chips are very strong in this game anymore.
The thing to remember about subsidies is that it's the consumer that pays them, not the carrier. Its just diluted into the monthly plan, Verizon would do the same. Switching carriers takes more effort than switching phones for many if not most people. I think the biggest concern for android is if the iphone crosses that bridge it becomes a head-on comparison of the two. At this stage My opininon is that the iphone would squeeze them out due to momentum.
The consumer pays the subsidy in the end, but the carrier decides which device gets the subsidy and how large it is. For example, you can't buy a Samsung Android phone full-price, then bring it to AT&T asking them to give you a $300 discount because you'll be using this device on their network for the next two years. That's essentially how subsidies work in other countries.
This gives the American carriers tremendous negotiation power over device manufacturers, who have no choice but to abide because there's no alternative avenue for selling their products.
(Nokia has tried for years to market unlocked phones in the US, with a resounding lack of success.)
It's not Apple's decision. Even if their AT&T exclusivity were expired, Apple couldn't just start selling a device on e.g. Verizon.
Consumers are trained to expect several hundred dollars worth of subsidies when "buying" a phone. The carriers control the subsidy, so they decide what devices go on their network. (Initially in 2007 Apple tried the waters with an unsubsidised $600 iPhone. That didn't work then, and competition is much more fierce now.)
The surge in Android marketshare is strong evidence that Verizon can do very well for themselves without Apple. I'm not sure if Apple's bargaining chips are very strong in this game anymore.