If you take as a given that the US wants to enforce their immigration laws, allowing people to attempt to break the law with a penalty of them saying "ok just kidding" doesn't make much sense.

It's like the liquids ban at airports. If you ARE caught with liquids, there's no penalty, you just throw them away. So it's not a disincentive to try to bring liquids aboard. Just keep trying until it works.

Each achieves the goal of keeping their intended contraband out, why make a big deal out of it? The angry boneheads at the TSA are not responsible enough to teach behavior modification.