Getting a shirt for free essentially halves the price for both shirts. So you're not looking at 5 equal shirts, you're looking at 5 shirts and one of them is half price. You just don't know which. This made me try to figure out which one people are least likely to buy (made easier with that star) and then evaluate whether the decreased appeal is worth the price reduction.

I guess that's the idea, but it's not a very positive experience for me. That's a lot of time thinking about the least appealing design.