Of course they dropped the red light infraction - it is a lesser included offense that established the reasonable suspicion for the detention in the first place, but they do not need to charge it.
Your attorney can always try to make a motion for an unlawful stop, but if they had ANY suspicion that you had committed a traffic violation, the stop was good. Heck, if you were unable to clear the intersection by the time the light turned red, that might be sufficient to justify the stop (depending in AZ law).
What does your attorney have to say on all of this?

