If there is a stop sign, and everyone ignores it, and nobody enforces any sort of retribution on the drivers who ignore it -- then what useful purpose does it serve? It's no longer a "Stop" sign, because it doesn't encourage anyone to "stop".
If 'everybody' ignores it, then sure it's meaningless. Except everyone doesn't ignore it, but a lot of people take the risk and do. And some of those get caught and get tickets.
Same thing with immigration. Not every Mexican runs across the border. Some do gamble that it's worth it and do. McKennedy would make a lot of those pay the fines.
The other guy's argument, presumably, was that the borders are meaningless because some get through them. So my question remains is whether the stop sign is meaningless if I ran it.