Remember the big scandal a few years back with all the yachts and sailboats that were confiscated because didn't have permits or permits that were expired? SAT/Aduana got a wild hair and decided to do something.
It's like drug raids and undocumented workers in the U.S. They don't go out on the streets everyday, but once in a great while they decide to change statistics, send a message, etc. and go out on the search.
Same thing here. But just like the yachts, people knew the rules, chose not to follow them, and then trashed Mexico when they got caught.