Having never owned or set up this kind of Christmas lights, I'm curious. What exactly is it that makes people think they need such "adapters"? Do they have female plugs somewhere? Why?
Christmas lights typically are male/female so that can daisy-chain the together to cover a longer distance. Rather than have to buy a 400 foot cable you would have 4x100, or 8x50. This is useful so you can more freely extend your decoration, or replace a smaller failed section. If you start with them the wrong way, you have to reverse all the cables.
Christmas lights often have a female plug on the end to allow chaining sets of lights together. A common mistake is to put them up the wrong way and have the male connector on the far end from your outlet, and the female connector right there. That's when such a cord starts tempting even reasonable people.