The letter was an insult to Spain. The writer complained they were busted in Mexico because a woman in his car was drinking beer and he said that she was a visitor from Spain "following an old Spanish custom."
Spain does not have such a custom. The legal drinking age in Spain is 16, but they don't have the youthful driver carnage on the highways because few youths own cars. They use sobriety checkpoints, seat belts are mandatory, children can't ride in the front seat, and helmets are obligatory for motorcycle riders.
The police have power to levy fines on the spot for violations, and they are not on the take. A visitor to Spain who tries to adopt this "old Spanish custom" could well encounter a Spanish reality--a trip to the slammer.