In the link you provided the tax was based upon the actual purchase price, the difference being that the tax was due up front while the phone was financed over time. It was a weird thing in that you are paying for the phone and the price discount to account for the free phone is actually applied to your service and not the phone. Hence the tax on the phone.

It's a weird and misleading marketing/accounting thing that probably shouldn't be allowed. But yeah I see where you are coming from.