There is very little risk in entering from Mexico. Ask the 5,000+ dirt farmers the sneak in every day.
Anyone with just a tiny bit of training can cross unimpeded at anytime.
A substantial percentage of these are caught or deterred. There is no risk for the "farmer" since he can just try again the next night but if a terrorist is caught there are huge ramifications for his network. And a twenty per cent chance of exposing these networks is far too high for AQ.