Would you rather be paid upfront or in arrears? Simply the timing of the payments is a really big financial incentive to have tiered-pricing - which is why you see it so often in the wild.
Couple of counter-measures for this exact point I have seen in the wild:
- companies letting you top up your account balance, then do usage based charges on the balance
- companies having a billing-threshold where they trigger a payment (or multiple) during the month