IIRC, pre-iPhone, US carriers would usually unlock a GSM phone after your contract was up, or even right away if you were a customer in good standing with them.
Then Apple came in. Their original revenue scheme, used for the first year, was to profit on each phone sale, plus take a cut of the monthly bill (in effect, taking the user's monthly subsidy stipend). AT&T didn't care... it was the same amount either way to them.
The only trouble was, if the phone wasn't on AT&T, then Apple didn't get that monthly contract kickback.
At first, Jobs seemed secretly delighted with jailbreakers. Their interest meant the iPhone was a hit. He would mention jailbreaking with pride and a little smile.
Then Apple's revenue stream started to be affected by a lot of unlocked phones being used on carriers other than AT&T, especially devices resold overseas By some counts, it was as much as 20% of all iPhones. You could see the change almost overnight in Jobs' attitude towards jailbreaking. Suddenly, it was not fun anymore. Jailbreaking became evil in his eyes, and Apple has fought against it ever since.
It also led to Apple dumping the monthly scheme, and going with the usual upfront subsidy model, starting with the 3G.
Edited by KDarling - 1/25/13 at 11:27am