| Old iPhone: You pay for a phone not subsidized by AT&T. You bring it home to activate it. If you don't, you jailbreak it.
Result: Either way, AT&T doesn't lose any money. Many people bought iPhones with the intent to unlock them, and who wouldn't subscribe to AT&T.
New system: You get an iPhone subsidized by AT&T. You have to activate it in the store, so there is no way to avoid it.
If they allowed you to take a subsidized iPhone home, you could unlock it. If you unlocked a subsidized phone (without signing the contract first), AT&T is throwing away money, since they're putting an investment in (by giving you the phone for cheaper upfront) but not getting anything from said investment (if you unlock it without signing a contract, AT&T won't recover the money they used to subsidize the phone).
It has everything to do with jailbreaking. The new system means they can subsidize the phone without having to worry that said subsidy will not be recovered. | 
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