The Supreme Court last week struck down the federal ban on legalized sports gambling as a violation of states’ rights—and that’s bad news for the gun lobby. In Murphy v. NCAA, the justices ruled that since Congress had declined to establish a national standard for sports betting, the federal government had no power to “command” states to prohibit it. “Conspicuously absent from the list of powers given to Congress,” Justice Samuel Alito wrote for the majority, “is the power to issue direct orders to the governments of the States.”

The...