No, I think it was rooted in the exclusivity deal. Looking back I think Apple wishes it didn't sign that crap (but there are its merits too). It was one of the first amazing smartphones. It was damn expensive back then and what not. Apple needed to get paid off big time. They needed to get paid so AT&T could offer a subsidized phone. Yeah even at $500 or whatever the iPhone was still not that expensive. The N95 at that time sold for $500 and the N82 that just debuted right after the iPhone was closer to $600. I think Apple later realized that pricing so high didn't work so they slashed prices drastically $200 was it? So it became $300/$400 for the iPhone 2G.
Anyway, the thing is the US phone market has the trait of exclusivity. So even if you want a cross carrier phone like the Captivate/Vibrant/whatevertheshit, you need 4 different models. Why? Who knows. I think this is the dumbest part of the US phone market. You're absolutely right that Apple would've had to design 4 different iPhones. Apple would probably want a SINGLE iPhone to do it all, but the carriers want differentiation. The biggest part of marketing for carriers is almost always phones. My phone can do this. It can do that, etc etc. Look at the EVO commercial, iPhone commercials, Droid commercials. Can you imagine 4 different flavors of iPhone commercials for 4 carriers? Now Apple's not going to design something radically different for the 3 other carriers. It's just like you can't design an iPod that works only for Macs and one that only works for PCs. That's retarded. You got one that does it all. Apple started with a firewire only strategy, but it moved to USB in the end as its a more universal format. The same goes with the iPhone. You really just need ONE identical device to be able to sell to all 4 carriers. Even if the internals are different with different antennas, the phone itself has to be virtually identical to the consumer. I think this is where carriers will NOT bite.
But look, honestly who cares? This is the US. This is one freaking market. Granted it has the most influence on Apple's strategy and vice versa but Apple can spread its phone across the world if it wanted to. It's interesting Apple doesn't just start selling worldwide and deployed across countries so slowly. There's got to be more to this whole withholding strategy