The questions I have: Are US Special Forces are going into Mexico to train Mexican forces to go after the cartels or is it a joint operation or are US Special Forces going into Mexico to kill all the cartels?
The mission is the training, until there is a new role reported we should assume that the training is it, probably training and intelligence and air/satellite reconnaissance assistance from other American elements.
Sometimes SF finds excuses to escort the troops they are training and do it against instructions but my guess is that the safety of the trainers would mean protecting them on base.