And the corollary to that answer is... Apple may very well make less money if they did so, because the carriers which are now willing to pay Apple over $600 per device compared to the market price of around $200 for the iPod Touch. The primary difference between those two devices, from an economic standpoint, is that a service is provided through the iPhone which can be locked to a certain provider. Without that ability, carriers would be willing to pay far less of a subsidy.
We can all agree, I'm sure, that Apple did consider the option of not having exclusive contracts, am I correct? While we all can sit here and fudge numbers all we like, can we also agree that Apple had access to data of greatly higher quality which led them to the conclusion that doing it the way they have done it was the best way to maximize expected profits.
Looking back at the results of Apple's decisions, it is hard to imagine that Apple is regretting them. Certain users who prefer Verizon or whatever other carrier they use definitely regret those decisions, but not Apple. Remember, many of the decisions were made when Apple did not have a strong bargaining position. While the device may have been nearly complete, Apple was a new entrant to the phone business and it likely took some very crafty bargaining to get as good of a deal out of ATT as they did, even though now looking back it might look like ATT got a great deal out of it as well.
That's economics. If you think Apple should have sold the device to everyone instead of just ATT or Rogers or whomever, no one is stopping you from creating your own godphone and selling it on the cheap