Thing is that bmx racing is so slow and the turns so easy they don't really need brakes. If there was a course with a 100m downhill into a concrete wall hairpin then every single bike would have a front brake. But when you never go fast enough to warrant brakes then you don't need them. They would only have them on the rear to aid turning by locking the rear, that is why they aren't on the front. They are more for turning than stopping.