Organized-labor headlines typically offer a zap of top-line shock — UPS is paying some drivers $134,000 a year? Philadelphia is paying a police detective $310,000 a year? — but those six-figure sums don’t capture the true cost.

As can be seen with the enormously costly backup at the port complex in San Pedro, Calif. — which handles about 40 percent of U.S. container-ship cargo — the issue is not so much high wages as highly rigid and inflexible labor practices.

The problem in San Pedro isn’t that the longshoremen are earning, on average,...