Hundreds of people die preventable deaths every year trying to cross that border. Exposure, dehydration, drowning, etc. If you don't know anything about where these people are you can't do anything to help them.
Even if you support open borders, this technology is essential.
The way this actually works is that militarization occurs most heavily in geographically favorable parts of the border, so that the least surveilled parts are the most physically dangerous to cross. In this way, increased technological sophistication at the border increases the number of deaths in border crossings.
Border Patrol isn't out there rescuing thirsty migrants. In fact, they're known for destroying life-saving water caches and brutalizing activists who try to provide water or medical aid to people wandering that part of the desert.
Nobody publishes statistics on this directly but we know that in recent years that the number of border crossers that have to be rescued from dehydration and heat exhaustion is in the hundreds per week.
Given that AST's towers/software identify objects of interest to CBP, it's probably safe to assume that number is high.
Even if you support open borders, this technology is essential.