Sales taxes in the USA are messy. Instead of being included in the prices as they are in most countries, they are added on at point of sale. The main effect of this is to allow companies to quote prices which are lower than what you really have to pay. A secondary effect is that it allows mistakes to occur.

It should be obvious to anyone with an IQ higher than that of a pet rock that sales taxes are supposed to be applied to money actually paid, not to money that might perhaps have been paid in some other circumstance. For those who have any doubts, it would be easiest to just read what the relevant state government itself has to say. E.g. Washington State says that when a store does not receive reimbursement from anyone for the difference between the usual and discounted price, sales tax is computed on the discounted price. They specifically say "Restaurant 2-for-1 coupons fall into this category."

Despite this, a number of US restaurants charge sales tax on the total theoretical cost of the meal before applying discount coupons. This is particularly irksome as there are only two possibilities for motive:

  1. The restaurant managers are too stupid too understand how sales tax works.
  2. The restaurant managers understand, but are deliberately defrauding the public.
Whichever the reason, they are collecting money in the guise of tax and then either paying it to the State when there's no need to, or they are collecting the money and then keeping it for themselves. It's the customers who lose out from this stupid or fraudulent activity.

Restaurants I can remember doing this include: