I know it seems illegal and you can thank the government regulators for it, but it has something to do with the telecommunications industry.
Depending on your state, for example I know that a California vendor has to charge tax on the retail price if the discounted price is more than a certain percentage. Texas has a simlar law.
If you think they cheated you, you should contact your state taxation authority to verify if it is the law.
Sorry, I know that this isn't an actual answer.