Another thing to consider is that sometimes (myself included) people buy items on sale or on clearance, intending to give them as gifts at a much later date. I have done this with clothing for little kids, but I stick to the basics (stuff I KNOW will get worn, such as onesies or simple pull-on cotton pants or tshirts). If Target accepted them as returns without a receipt, Target could be stuck with merchandise that's several months old, and have to immediately mark the items down far below their original price.

I have stopped doing this (mostly because I end up forgetting I have items tucked away), and instead, I buy things for no occasion and give them away as I buy them, which means I may not buy something for a kid's birthday (which shouldn't be a problem, as they get things from me throughout the year, and they should be taught to appreciate that fact).