Yes, you are buying time (data) and paying for it whether it is used or not. For this reason I am post paid. I buy the cheap package $14.99 and only have went over a couple times, however, when I go over I can call in and change the plan or add time and it's not as costly. Most times I am within my allotted amount, which is why I take the smaller plan, not the 2GB plan.
Its a service just like cable. You don't pay for cable only when you turn the box on right?