It seems to be a regional thing in Mexico about using the left turn signal to let you know that it is safe to pass, it is also not allowed to block traffic from behind while waiting to turn left if there is a parallel frontage road or spot to pull off on the right. I usually stop and wait unless I can clearly see ahead.
As my insurance agent told my sons when they started driving: When you see a turn signal activated on an on-coming vehicle, it only means that their turn signal works.