All their plans have that structure. I'm guessing that they want to be able to plan what resources to give to Alpha on the basis of what their customers have si

All their plans have that structure. I'm guessing that they want to be able to plan what resources to give to Alpha on the basis of what their customers have signed up for, so they want to discourage "overuse". Also, you pay up front for the quota and presumably pay in arrears for any extras, so obviously they want to encourage you to pay earlier. (Still seems pretty daft to me, though.)