You are right. Apple set their contract up with at&t from the very beginning to determine their own price scheme for the iPhone. They then allowed carrier subsidies with the release of the 3G, but they still determined the price. With the smaller carriers that are selling the iPhone $50 cheaper than at&t/VZW/Sprint, Apple may have been more lenient with to let them sell it at a lower price point, maybe to keep competition healthy.
And even if there were no phone subsidies, the original iPhone sold like crazy at full price, so even a slightly higher priced iPhone with LTE under the hood would probably sell out in Pre-order stage too.