Would you advocate the border control should point guns at anyone who crosses the border? Or is that a needless risk to the safety of humans?

I'd ask you to consider for a moment, exactly what is going to happen to that mine? I assume you thought it would damage or destroy any anchor dragged across the cable? Well it's really hard to predict how much damage it would cause but lets assume it works exactly as you hope and say it only destroys the anchor chain of the boat. What if there's a storm, and that ship can't anchor safely, but because it's run by incompetent, or evil people they don't care, and try to park near another ship crash into it. Both ships sink and a bunch of innocent people on the ship that just happened to park in the wrong place, die. That's assuming all the people on the "evil ship" were involved and weren't just hired to work delivering cargo.

You admitted it was a stupid question, but I'd like to suggest you should avoid defaulting to suggesting death and destruction. A better stupid question would be "why can't we armor the cables" instead of "why can't we hurt other people"

Edit: or do you mean mines anchored to the cable on the sea floor? That would make it easier to kaboom the cable instead of cutting it.

I imagine there must be some other deterrent measure but drawing a blank. My brother was the navy man...