So, you have a border that is not widely perceived to be a very big security problem, and most national security incidents have arisen through other vectors, and you propose redundant, expensive measures for border security. This raises the question of why, since "border security" doesn't seem like a realistic answer. The people who see it as oppressive believe the real purpose of this push for "border security" is a political statement to the effect of "brown people aren't welcome here." (Trump also kind of helped give this theory currency by claiming that Americans of Mexican descent are so likely to be biased against him due to the wall that a Mexican-American judge needed to recuse himself.)