In the US, you should absolutely NOT be paying sales tax on gift cards. They are not products for sale, merely an alternate form of currency accepted by individual or multiple stores. If you get charged sales tax on the gift card, then get charged sales tax on the item you buy with the gift card, you are essentially being taxed twice on one purchase...double taxation is illegal in this country (unless you're talking about inheritance...in which case double taxation is allowed for some reason).
Getting charged sales tax on a gift card is like going to the bank, asking the teller to break a $20 and only getting $19.50 back because of sales tax.
Let's look at an example: You get a $10 gift card at a store as a gift for Minerva. You get taxed, and have to pay $10.50 (5% rate). Minerva goes to that store, purchases something for $20. Tax is added, so she's paying $21.00. She uses the gift card, and has to pay the balance, which is $11. So the total paid is $21.50 ($10.50 from you and $11.00 from Minerva)...which means that the final tax on that item is $1.50...a 7.5% tax instead of the 5% it should be. That's illegal.
In order to charge tax on the card itself, then the store could only charge tax on the balance of the final sale...meaning that Minerva would take $10 off the top, be left with a $10 charge, to which tax is applied...equalling $10.50...plus the $0.50 you paid on the initial card. Then the tax would even out. BUT the problem with doing it this way is that each state has a different tax rate, so if a gift card is acquired in one state and used in another, then state #1 is getting sales tax for an item sold in another state...and state #2 is missing out on some of the sales tax generated. It's a mess.
So no tax is charged on a gift card. Tax is applied to the final sale so it only gets applied once and goes to the correct state.
So to make a long story short, you need to go to Target and talk to a manager to get your money back.