given the essentially unchecked flow of immigrants
coming across the Mexican border every day.
The words missing there are "poor Latin" (immigrants). Coyotes are not stupid and want to be left (more or less) alone and free to conduct their business. They "self-police" themselves by selecting who they carry across the border, choosing:1. People they can trust not to create problems along the way
2. People that will try to stay out of trouble in the US (if you're successful, you're more likely to try to get your family in as well and recommend the same coyote. If you're caught (specially early on) you might turn them in.
3. People that don't look suspicious (that might be undercover law enforcement, for ex)
And above all, they KNOW that the day a terrorist attack within the US is traced back to them public outrage would make law enforcement come down on them with the heat of 1000 suns and the border would be closed tighter than Scrooges coin purse.
Smuggling in illegal aliens is illegal... but smuggling in terrorists is illegal, dangerous and extremely bad for business.
The same thing applies to drug smugglers as well. Collaborating with terrorism isn't in their best interest.