Well, because you're trying to compare apples to oranges... Unlimited costs most of us $30/mo and some lucky ones $10/mo. Tiered data starts at $60/mo for 2GB, and yes it gets cheaper per GB the more you buy, but it is still way more than $30/mo for unlimited. You can't just say, well if you have the 10GB plan it's only $10/GB. It is still and extra $40/mo MORE than what I'm paying now for unlimited...
Your argument that talk/text is less on tiered is moot. I pay short of $80/mo for my plan w/ discounts as long as I talk less than 450 minutes/mo (unlimited texts) which is not an issue for me. Yes, I could be paying the same for a tiered plan w/ 2GB, but I use a little more than 2GB so it would cost me more for the 4GB plan.
Our apples to apples price point comparison is with the plan that would match what we are paying total per month now, which is the 2GB plan. In that plan you pay $60 for 2GB of data, hence the $30/GB comparison. Any plan with more data is more expensive, period. Doesn't matter what the $$/GB is...
You decided to take on the roll of devil's advocate, so don't get upset when your points are counterd...