The AT&T deal isn't completely understood by many. The iPhone required a number of network changes and modifications to work. At the time, Apple was not a cell phone manufacturer and it was hugely risky. Several carriers refused to play. AT&T was willing to make changes and invest themselves in a big gamble.
Consider what would have happened if it did fail. Both companies would have lost huge amounts of money. Apple also had a poor track record with touchscreen devices and phones. The Newton was a failure. So was the Motorola ROKR phone.
The iPhone ended up a successful gamble, so it wasn't unreasonable to ask for exclusivity. Also consider that Apple could make much more money now by not being exclusive to a carrier. Apparently, AT&T is already worried about being dumped by Apple so they can get a bigger marketshare. How that shakes out in about 30 months will be interesting. I have no particular love for AT&T, so my money is up for grabs.
Another important consideration is that Apple is a publicly traded company. That's important for two reasons.
First, Apple has a duty to shareholders to maximize profits and protect their IP. If they blow it, shareholder suits are expensive (possibly upwards of $1,000,000 a month in legal fees), there's a lot at stake if they lose and the existence of a suit drives off potential investors. It's the same reason why other companies do the same, but most of those have lower profiles and don't attract the same level of mainstream attention.
The other point is that anyone can buy shares of Apple. If you think Apple is making a killing on profits, you can get a piece of that action yourself. If people are brainwashed into paying too much, then head over to your broker and cash in. The more money Apple makes, the more money you will make.
