It is not legal. Apple and AT&T with the iPhone have conspired to release features little by little to increase interest and sales of the iPhone. This is one of the features that has yet to be released and it costs an additional $35 on your bill on other phones with which it is available. There have been workarounds, but they are not legal, and they will void your warranty, although it is generally possible to restore the iPhone to factory specifications fairly easily.
More than that I will not say, other than that it does strike me as odd that an "unlimited data plan" is not at all actually unlimited. The problem is that under existing conditions iPhone use has been more than was projected and caused the AT&T network a lot of trouble as it is. The iPhone is really their bread and butter and the only reason they are competitive with Verizon. They have actually filed suit against Verizon about those "there's a map for that" commercials. Some people erroneously conclude that the map show is the total sum of all AT&T coverage and it is not. AT&T has 2G coverage nationwide and is in all the places Verizon is.
I have an iPhone myself and I can attest that the 3G coverage is good where I live, but to conserve battery I turn it off at times (the battery being the Achilles heel of the iPhone). I can't honestly say I notice a huge speed difference. The one feature it make me mad that the iPhone doesn't support is an adequate voice dialing program which can be used with a bluetooth unit. The apps that do exist require one to start the app on the phone which requires one to unlock the phone, and look at and operate the touch screen. Kind of defeats the purpose if you use it to be safer while driving if you ask me. Fortunately they do have voice dial as an AT&T service and I do use it. The only trouble with that is that it uses minutes, it costs me an additional $5 a month, and I have to always be sure that it is the last number I dialed when I am in the car. Other than that it works quite well.
The trouble with Verizon is that for the most part they have boring phones in comparison the AT&T in my opinion. Overall I have really liked my time with AT&T.
I had to have an iPhone 3G when the price came down to $99 (8GB model), and I like the phone, but like most Apple products it can be annoying. I see people slamming Micro$oft all the time for being a money grubbing evil corporation, but in my opinion, M$ could learn a lot about chiseling the consumer from Apple.