They don't anymore for new hairsytles. But for why they did locked hairstyles is that they can design a hairstyle just for one model, not have to worry about what it will look like on the others, make it look really good, and it lets them do a more variety in hairstyles.
Example is one I already mentioned. that Au ra and miqote do not have human ears. If they make a hairstyle that exposes where the ears should be, it will look creepy. SE draws your attention away from their "missing ears" by giving them hairstyels that cover that area. Now if you make it a requirement that hair styles are all available to all races, now every race has to have new hair styles covering the ear area. This lowers the variety in hairstyles.