doesn't give me the same rights/responsibilities as law enforcement personnel.
Law enforcement is under pretty similar rules. In most states they can not shoot someone out of hand to protect ordinary property. There must be a threat presented in order for them to be allowed to shoot.
There are other ways of stopping a smash and grab robbery without using deadly force. However I would think that an unarmed thief is taking a big risk that some unknowing and well armed shop keeper won't know or won't care.