You know unless there is a clear marking on the discount that says "off retail" then they did nothing wrong. Lots and lots of places discount the price after tax is applied. It's a chickensht way of saving a few bucks here or there. But unless the offer says "off of retail price" or something similar, it's totally legal.
Check with the rules established by the state comptroller. Corporate offices don't establish how the state collects taxes. In Texas you are traced on the total of the discounted price. If it is a coupon then you are taxed on the full price then deduct the coupon discount. So, it is actually illegal.