Because I used 2GB without even thinking about it this month. And thats before iCloud and photo stream and the like were fully implemented. All while on wifi half the time. If I were to go to the tiered plan that could potentially add a lot more than $10-20 per month to my bill. More like $40+. And that's just in the near future. What happens next year when my data usage goes up? And you know it will, everyone's will, it's the way of the future...
Edit: Also, I'm on unlimited, I shouldn't have to agonize over every thing I download. That's the point of an unlimited plan. If I have to count mbs, I'll miss out on something awesome on the internet...
Next year, when your usage goes up... you'll pay more. Just like if, next year, I get a bigger TV and run the heat more in the winter, I'll pay more. If you can't afford it, you'll have to come up with a data budget and start actually thinking about how much you're putting through the airwaves instead of blowing through 2gb without thinking about it. If Apple made iCloud so that it sucks a ton of data, well, it's going to be an expensive feature for people who don't use wifi. Life is hard sometimes, but it's not up to AT&T, or any other telco, to make Apple's decisions work on their dime.