Is this activity considered illegal?

Its dishonest and you likely get the people you trick fired. Gift cards are designed so companies can get their money regardless on whether or not you use them or not. I do not think it is "illegal" but think about it, if you think something might be wrong, it probably is, The fact that the stores often refute your attempts is even more proof that what you are doing is wrong. It's similar to card counters in Vegas. Technically, they are not cheating, but they are taking advantage of the casinos. Your not going to like my answer I'm sure. So, in short, it's not illegal, just dishonest.