Why are Mexicans from Mexico so rude to others?

I have noticed that when you go into an establishment such as a tacqueria or even a department store that has a lot of Mexicans from Mexico working there they go out of their way to be rude to non Mexicans. They will be as nice as pie to the customer in front of you while they are all talking Spanish and then once you come to their line their demeanor totally changes. The worst place I have seen this is at MacDonald's. It is unfortuante that MacDonald's seems to employ ONLY non English speaking workers from Mexico and the women have a particularly strong attitude towards non Spanish speakers. One last item of note, and something I have discussed with my Mexican American friends is that Mexicans from Mexico are very rude and hostile towards American born Mexicans. So why is this? It seems that as long as we are not being rude to them they could show some courtesy towards us since they are in our country and not the other way around.