Ben, your alarm is understandable because I was the same but I spoke to a guy from ReBall at campaign at some length about the matter.
Basically what defines legality is the amount of pressure in joules that a shot exerts on Impact. Along with being softer than paint balls and lighter, they are designed to be shot at a velocity of no more than about 230FPS. This way they fall within the law as they do not exert more force than a regular paintball. Apparently it has all been checked out and given the thumbs up and the guy i spoke to seemed to be very aware of what the laws were.
I've been shot by one at about 20 yards, at a velocity of around 250-260 ish (didn't adjust the gun for smaller size of balls) and it didn't hurt any more than a regular paintball. They also make a rather different sound during flight as they are on their way to impact.
I was/am VERY impressed with the ReBall. As soon as I can get the cash together I'm gonna be buying a load and using them for indoor training in a similar sort of venue as Joy use.