THE DECISION of President Carter to veto the Department of Justice appropriations bill is right. The anti-school-busing rider that Congress attached to it is a fraud. The rider has been sold to a majority of the members of Congress as a way to stop the busing of children in school desegregation cases. But that's not what it is. Even if the rider should become law, the buses would keep rolling. Judges would retain their power to direct that students be bused in situations where they saw fit to, and no outstanding judicial order would be changed.

This particular rider is much more subtle. It is an effort to retire the Justice Department to the sidelines in just about any school desegregation dispute that gets into court. Not only would that put the legal and financial burden on private individuals. It would also deprive the federal judges of one of their primary sources of help in difficult cases. The lawyers of the Justice Department have provided the courts with invaluable aid -- not always in favor of busing as a solution -- in school cases for many years.

Because the rider attempts to keep those lawyers and, through them, the president from participating in these cases, it may well be unconstitutional. The president has an obligation to see that the Constitution and the laws are enforced. By ordering Justice to stay out of any case in which school busing may be imposed as a remedy, the rider orders the federal government to stop trying to enforce the Constitution's equal protection clause. In that sense, it is as if Congress were attempting to pass a law telling Justice not to enforce the bribery statutes unless the judges agreed in advance not to send those convicted to prison.

Congress should not be in the business of sending a phony signal back home that its members are trying to do something about busing when they are trying to undermine the role of the federal government as the defender of everyone's civil rights. It can do its own reputation, not to mention the law, a favor by sustaining the president's veto.