Biology was always a mystery in college, but I have also felt that this was obvious. They look very similar for heaven's sake! My paranoid self thinks shrimp, crabs and lobsters are labeled differently from bugs, so people can eat them without being repulsed.

Compared to the alternative that, IIRC, insects are closer to myriapods? Crustaceans aren't the only crunchy animals, even after this reorganization.