Legal logic.
You are not "buying it" if you are taxed for it.
Believe it or not, they use that logic to justify some really outrageous things that the government does in the name of the public, like war.
If you are taxed for it, then you have no say in what you get, and have no recourse in court.
If you buy it, there is an implied agreement and you have recourse in the court.
Cute, isn't it? Another building block for the Empire and the Dictator.