As stated by /u/Rednekked, not sure where you are, but this has been an issue in Canada for some time now. Thing is, it has nothing to do with tax laws (more on this in a moment) as it does with an error in POS coding.
Canadian tax law states that no sales tax of any kind can be applied to the purchase of a gift card (not to be confused with "service fees" charged for the purchase of pre-paid credit cards; those are allowed under different rulings altogether, but back to the point...).
Some retailers, however, don't code PSN cards as gift cards (even though they ARE GIFT CARDS under the law). Instead, they treat them as digital purchases, which is incorrect, and leads to the purchaser being taxed twice. Once on the card purchase, and again on any purchases from PSN itself.
It's more a matter of ignorance on the retailers' part. They see a digital purchase card (guess the best example is PSP digital code cards) which they can charge tax on, then simply assume a PSN card is the same thing (it isn't).
I ran into this issue myself. Went into a Rexall (drug store) to pick up a prescription, noticed PSN cards on their gift card display, and figured I'd pick one (yay, convenience). When they scanned it at the cash, however, I saw they added taxes and cancelled the transaction immediately. Didn't bother arguing with the cashier (not their job to know or care), and instead emailed their head office when I got home. Got the run around about it being a digital purchase, blah blah blah. Gave them the run down on tax laws surrounding gift cards, but still got stonewalled. Took it up with Revenue Canada (I believe, may have been another area, it's been a while), and they confirmed Rexall were in the wrong. Forwarded the info I received to Rexall, and got a nice apology. Never bothered to see if they changed their POS practices though, just went elsewhere.
Can't find the links/story anymore, but this same issue was talked about at length when it was first discovered Futureshop had started charging tax on PSN cards. After enough complaints, and possible legal action along the way, they finally sorted it out and no longer apply taxes to a PSN card purchase.
Sorry for the long-winded reply, but wanted to share my experience with all this. It's this kind of stuff that makes life harder for consumers all the time. Sales taxes (especially in Canada) are a trumped-up bunch of nonsense to begin with; nobody should have to pay double.