My understanding is that they are death traps designed to pull you under into a net and then you drown. There are warning signs on the US side but not the Mexican one.
Not sure how they work as a deterrent, maybe the goal is to kill enough people so news goes back to the migrants' countries and scares the others off the trip? Otherwise if you already made it all the way to the border I don't think you'd turn back because of them. Then again maybe the point is simply to kill migrants.
And if this were literally the only potentially lethal hazard on these poor people's trek, it might actually work as a deterrent. But when people are fleeing for their life, through lethal hazards literally every day of the journey, only to face lethal risks after actually settling in America? What's one more risk, when the penalty is always the same...