According to the Associated Press, Springsteen—who is notorious for extremely long concerts—had already gone a half hour past curfew when McCartney came out:
Springsteen had already exceeded the 10:30 p.m. curfew by half an hour Saturday night when he welcomed McCartney on stage and the pair sang the Beatles hits "I Saw Her Standing There" and "Twist and Shout." But the microphones were turned off before they could thank the crowd, forcing them to leave the stage in silence.
A statement from concert organizer Live Nation said it was unfortunate that Springsteen's three-hour-plus performance was stopped "right at the very end," but it said that the curfew had been laid down by the authorities "in the interest of the public's health and safety."
Afterwards, the E Street Band's longtime guitarist Steven Van Zandt vented his frustrations on Twitter, calling England a police state.