Its all about selling phones. Unlimited data was the cheese in the mousetrap, and most people were unaware of how much they use, or whether they'd use massive amounts with an iPad. If they were afraid of using unreasonable amounts, the iPad would be bricked on their shelf. Now the marketplace has decided its not a flop (the iPad), and the sales people can talk intelligently about how much the average consumer uses.
Which, since they're a business, they'll draw the line on the cheap side of average, and be able to upsell the packages. Average customer uses 2GB, so you can buy that...
And if you go over and really love using this awesome product you might want to get the 5gb plan - because otherwise we'll charge you [$x>.0025/mb~/day/COMPLICATED! MATH!]
I'll take the bigger plan, please, because you're harshing my buyers buzz, while I stand here with my girlfriend...