They use fake SS numbers to get on-the-books jobs or they work for cash in jobs that aren't at the same location every day(construction, harvesting). Enforcement at the business level is not practical compared to just a border to monitor. Do the math, LOL if you're capable. You're talking about millions of government agents to handle the huge numbers of businesses compared to watching the border. We can fly drones for hours with infrared vision to spot them.
Seriously, you are devoid of any logistical logic. You must have a very limited set of skills, because you don't know shit.
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