How would either of the two principal characters argue that it is perfectly reasonable for them to steal horses or cattle south of the border whereas anyone doing likewise 'this side' of the border merits summary execution?
I beseech ye in the bowels of Christ, think that ye may be mistaken.
Nobody in the film was summarily executed for stealing horses or cattle. They were executed for acts of murder, or for being an accomplice to murder.
shareOP, I think the fact that they were talking about two different countries made a technical difference. In a way. They pretty much followed the laws (that they agreed with) in the US; but perhaps felt no obligation to follow the laws in Mexico.
Also, it seemed to be a bit of a tradition for the guys to go change Old Pedro's horses north once in awhile and accepted that Pedro did the same to them once in awhile. Almost a friendly rivalry, if you will.
When they went into Mexico after horses, they usually went to Pedro Flores' ranch. It was a big place, and losing a bunch of horses didn't really have a life or death impact on anyone there. They weren't stealing some poor Mexican farmer's way to travel or make a living in the fields.
One of the reasons horse theft was a hanging offense in the West in those days is that horses were vital to economic and sometimes security survival. Horses were transportation (to gain access to vital services such as getting a doctor's help from town, trading goods and services), horses were a means to getting the farming or ranching work done so that your livelihood was not endangered; horses could be a means of getting away from danger (outrunning attacks by Indians or whatever). But this really wasn't the case with running horses back and forth with Pedro. Often they were pretty much just stealing back their own horses.
Thanks for that, guys. I was curious about whether people were still being legally hanged for horse stealing in the West at the time of Lonesome Dove (1870s) and the answer seems to be 'no' - http://boards.straightdope.com/sdmb/showthread.php?t=700666 -
In looking over the Espy file of legal executions in the U.S., I didn't find any cattle rustlers who were hanged, but a few dozen people have been legally hanged for horse thieving. Contrary to what you might expect from watching Hollywood westerns, most such cases were from east of the Mississippi and before 1810. The most recent case that I turned up was from California in 1851. James "Mountain Jim" Wilson and "Dutch" Fred Salkman were hanged for stealing horses on November 28 in Stockton.