I am not the one to try to explain laws in Mexico. I posted about an incident I was made aware of and added to it past information we learned on this forum about left turns in Mexico. I think at least part of the reasoning has to do with not holding up traffic while waiting from oncoming traffic to clear. I don't really know. But Mexicans grow up with this so they know this is the way you drive in Mexico and we need to be aware that sometimes there are going to drive in ways that are unfamiliar to us and in some cases what may seem weird to us and normal to them means we are the ones breaking the law and not them. It does not matter if we agree with the law or not. It is the law. We need to be aware and to try to adjust.
We also notice when we drive on roads that have a shoulder the slower traffic will pull to the right and sometimes put on their left signal to tell traffic behind them it is safe to pass. Some vehicle drive the shoulder much of the time. We have adjusted to this idea and drive the shoulder whenever it is safe to do so - watching for pedestrians and animals and rocks and various types of bikes on the shoulder. ( Hi Kate - if you read this - you know who you are).
Passing in Mexico on some roads uses this idea for passing as vehicles traveling in both directions use the shoulder to allow faster moving vehicles use of all lanes for passing. You may see a sign like this ....
Permite rebasar; utilice acotamiento
Translation: Allow passing; use the shoulder
This means what it says - use the shoulder to allow passing. When we pass we are hoping the oncoming traffic knows this too. After miles of traveling in Mexico you would think everyone would know this but sometimes it seems the oncoming traffic is either unaware or wants to play chicken with us as we try to pass a slow moving truck. Turn your blinkers on and flash your headlights as you proceed to pass.
We also know there are a great many roads in Mexico that do not have even the slightest hint of a shoulder - obviously no shoulder can be used for any of these rules. But if you are in a passing zone and there are cars behind you and you want to make a left turn you should be aware that some of them are going to try to pass you even if you have your left signal on.
Hope this helps understand another of the mysteries of Mexico.