No, he was foolish to believe that the US Government had the slightest idea about what it is doing vis-a-vis the US-Mexican border and regulations concerning the citizens of two countries in the others territory.
Here is a fairly comprehensive recap of the story: " Gun that landed Marine Jon Hammar in Mexican prison was legal, says veteran guide"
The "Official" word is here: "Guns are Illegal in Mexico"
My guess is that the US Consulate webpage was added AFTER L/Cpl Hammar's case went big in the press.
Some think otherwise. "http://www.mexonline.com/huntmex.htm"
"A Maximum of 2 Rifles or Shotguns per hunter is allowed, guns may be accompanied by no more than 2 boxes of American Shells per gun (50 shells per gun). With the proper hunting permit, additional shells may be purchased in Mexico through various Licensed Sporting Goods Dealers. Only .22 caliber bullets and 12, 20 and .410-guage shotgun cartridges are available. Cost is somewhat expensive, but when measured with the risk of going to prison for smuggling shotgun shells into Mexico to save a few lousy dollars, the cost is cheap.
Do's & Don'ts
Do use Common Sense; remember you are in a foreign country, with their own set of laws. It's not important, if you think you are in the right or not.
Remember, you're guilty till proven innocent in Mexico.
Do follow Mexican Laws.
Do carry your passport, hunting license and gun permit at all times.
Do be patient and respect the Mexican people, they intern, will respect you.
Do double-check your car or truck for extra ammo or unlicensed weapons, prior to your entry into Mexico. Most people get into trouble for this single reason.
Do watch where you walk. Rattlesnakes in Mexico are large enough to kill a person.
Do remember to check references.
Don't be stupid. If you have trouble differentiating what "stupid is" in the United States, that should raise a "red flag" to not attempt to hunt Mexico.
Don't even try to smuggle unlicensed guns or ammo into Mexico. It's not worth the penalty. The Mexican prison system is full of stupid American's, wishing they had been smarter.
Don't walk in high grass without snake leggings on and watching where you step. For some reason, in 20 years of Hunting Mexico, I have never seen a small rattlesnake. "
It only gets worse as you go South and get into warmer country, and finally you hit Mexico. Warmer climate, more game, sharper fangs ~ looking up some lists of poisonous snakes in Mexico and every genus has a representative there, and every species has one or more subspecies lurking around.
And lots of them I've heard!