I seriously doubt they are going to harass private individuals bringing back a few pounds of shrimp. I could be wrong, but this measure is intended to stop any commercial trade in Mexican shrimp, which is where the real dollars and cents are.
There will be a big stink at the border at the border if they do start confiscating shrimp. Literally. As the temps start hitting the 100 degree mark, exactly where will they put the confiscated shrimp? In the trash? Ewwwww! Such a wonderful aroma!