While the vehicle is stopped for this pretextual purpose, the cops may search the car even after the business of the traffic violation is taken care of. In this case, there was no traffic violation per se, but there was a potential crime - i.e., loitering, so the pretextual rules would still apply.

Once the car is stopped for reasonable suspicion, road block or pretextual purposes, the police officers may search any part of the car where they reasonably believe contraband may be hidden.

For instance, if they are searching for a body, they may not search in the glove compartment, but if they are searching for weapons, they may. In this case, with the robbery having been committed, they could have reasonably been searching for weapons, so it was perfectly legal to look in the back seat and see - in plain view - the weapons and take them for evidence.

In conclusion, the general understanding is that a police officer only has the right to stop people suspected of being involved in criminal activity. This area of police conduct is often found under the heading of stop and frisk rules. Basically,...

What this means is having long hair may not be a good enough reason to be "stopped," but walking back and forth in front of a bank with a large, bulky item under your jacket may be a good enough reason for a police officer to stop and detain you for questioning. In this case, they were walking around in a high drug area after a robbery.
Second, assuming the "stop" is valid, a limited search for weapons is justified if the officer reasonably feels in danger. Basically, this entails a "pat down."

Third, any search may only be confined to find objects capable of being used as weapons. For example, if the police officer goes into one's wallet looking for a weapon, this may not be the proper place for the officer to look for weapons. If the officer finds illegal drugs, then one may have a shot at winning a motion to suppress, if one are charged with possession of those drugs.

Bibliography

Barbri Multistate Review. (2004) Criminal Procedure. New York: Barbri.