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:
Restaurants I can remember doing this include: