The Avengers Why dont they bother keeping an eye on Bruce Banner

So, SHIELD takes the trouble of taking their top agent, Natalia, off a vitally important mission (right in the middle of a dangerous situation too, and violating what appears to be Russian sovereignty with the threat of blowing up the building with missiles) and sending her to India with a full squad of heavily armed soldiers in order to ask Bruce Banner to come in. They ve also spent millions researching and actually building a cage for his alter ego. They know exactly what Hulk can do. Yet once they ve got him onboard, the top secret, state of the art HeliCarrier, they let Bruce casually wander around with no restraints, and no one watching him free to be shocked by Tony Stark into potentially Hulking out. Guess they thought Banner was a really cool guy once they met him in person.