They don't know, nor is it in any way their fault that the CD only contains 2 or 3 songs that you want to hear.
Rather then complaining about such things, at the end of the day you are 1 person that can feel good about yourself knowing that instead of downloading, you bought the album and directly supported the girls with your purchase.
Also keep in mind that without a doubt there are people who don't buy mini albums, and instead wait for albums such as these to get all/most songs. So even if in your case the album only has a few songs/new songs, for others it might be a album full of new songs.