Not at all the case. It's intended value is to reduce the taxes paid and as such it is rightly not included in the calculation.BUT IT'S VALUE IS REDUCED BECAUSE IT'S TAXED!!!
We seem to be saying different things. You're saying the prebate can be spent and certainly it CAN. It can also be saved and not spent. Either is beside the point of the use of the prebate in the Calculator where it is intended to reduce taxes paid - it's conceived use as a tax rebate - so that an accurate comparison between the two systems can be made on an even basis.
It isn't an "error" at all but, apparently, a use which you do not understand. If anything, forcing in into spending would be an error from the standpoint of the Calculator since that would increase spendable income with no offsetting increase in spendable income on the income tax side thereby biasing the results in the Calculator.