But I bet they make a hell of a lot more sales, and a lot more profit, it's not like they are selling a physical product, it's digital and costs nearly nothing to upload again.
But you make the assumption that people wouldn't buy it at full cost if it wasn't discounted, gamers have a long, long history of paying whatever they are told to for whatever product is being flogged to them.
Some of the steam xmas sales were very out of character for some of the big publishers in terms of offering low prices, so I wouldn't be surprised if it's coming to an end.
Publishers know full well they could not have any xmas sale at all and still sell their triple A games by the bucketload.