Since most of these laws are written by politicians and not economists it's more likely that they want to appear tough on immigration to avoid "dey took our jer

Since most of these laws are written by politicians and not economists it's more likely that they want to appear tough on immigration to avoid "dey took our jerbs!" style anger from the electorate. It's sort of like how clubs force people to wait in line outside even when there is room. If they let everyone in, it might start to feel crowded and less fun.