On education policy, appeasement is about as ineffective as it is in foreign affairs. Many proponents of school choice, especially Democrats, have tried to appease teachers unions by limiting their support to charter schools while opposing private school vouchers. They hope that by sacrificing vouchers, the unions will spare charter schools from political destruction.

But these reformers are starting to learn that appeasement on vouchers only whets unions appetites for eliminating all meaningful types of choice. With voucher...