It isn't a law. It is the experience of the Who tragedy in 1979 when almost 18,000 tickets were sold as GA and 11 people were killed in Cincinnati. Cincinnati banned GA or festival seating and other cities limited its use.
Now, they limit how many tickets are sold, they require wristbands for entry to the GA areas, the open the doors about 1.5 hrs before the show. Stuff like that to control the crowds.
I don't think the ellipse has much to do with anything other than being a different way to interact with fans. The only other thing would be that the double GA area might allow selling more GA tickets than would normally be allowed because of the split areas.
In the UK, is the entire stadium GA or just a small portion of the tickets?