Because if public video surveillance was only used for legitimate purposes, worked well and made everyone‘s life better as a consequence there wouldn’t be as much opposition to it. In practice this is not always the case.
If every new proposed law to combat „child abuse“ was well intentioned and actually worked, there wouldn’t be much opposition either. But since they are mostly an underhanded tactic to censor the internet, there is.
So to use this legitimate actually useful example of fighting illegal immigration leaves a very bitter taste.