It is cut and dry. You carry a concealed weapon in the state of NY, Bloomburg will see to it your ass is doing a mandatory 3-5 years. The Giants were not going to immediatley cut him knowing this. It would have been a knee-jerk reaction and publicly frowned upon.
My opinion is the the Giants never gave any real indication that they were sticking by Buress to begin with. The probably knew all along that they were going to release him once they became aware of the severity of the charges facing him.