The FA threw away the right to have this as solely a football ground when they spent £800m. That has to get paid off, and the only way it gets paid off is to fill it up - hence why the NFL comes over, hence why gigs are played. It is poor timing to have England play around a week after the American Football game, but again, if there wasn't such a need to pay off the substantial cost, then the FA could hold a lot more negotiation strength.