Last week, the Trump administration announced it was quitting the global treaty that governs mail delivery, calling it unfair to the U.S. Another episode of Trump vs. the World? Yes, but in this case probably justified—and therein lies a lesson.

President Trump’s frequent threats to tear up the pacts governing U.S. relations with the world are often met with a full-throated defense of the global rules-based order. Yet many of the pacts that make up that order have features that disadvantage the U.S., sometimes for reasons...