And I'm not sure it applies here. It's not like people had to buy an iPhone. Yes if you originally wanted an iPhone you had to buy one locked to AT&T, but there were lots of other phones out there at the time.
The fact that the iPhone doesn't have a majority of the market share today should be enough to prove that the original contract did not hurt competition. Actually if anything it limited the number of iPhones out there since originally people on carriers other than AT&T couldn't even get it.