Hey, I'm not agreeing with them, just citing their regulations. And if the border groups wander onto Park Service land and some ranger has a bone to pick, they're gonna have problems.
If I recall, the perpetrator of the restaurant (can't remember which chain) murders in Texas some years ago had been arrested at the Grand Canyon, drunk in his trunk with a loaded pistol in his glove compartment. Got off with a ticket for $25 or $50. Bet these guys don't get off that easy. They're not only stupid laws, they're enforced selectively.