Punishment meted out this week against several FBI agents who exceeded their authority while investigating a group opposing the Reagan administration's Central America policy should send several strong signals to all Americans.

For starters, it reaffirms that Americans have the right to free political dissent and assembly; that they are free from guilt by association; and that in a democracy, police powers are not absolute.Six mid-to lower-level FBI employees were either suspended or censured and a seventh resigned after negligently operating what FBI Director William Sessions told Congress was an unnecessarily broad terrorism probe into the Committee in Solidarity with the People of El Salvador (CISPES).

Sessions said top-level officials had been unaware the bureau was collecting information on rank-and-file members of the committee who, he said, had nothing to do with international terrorism.

The CISPES probe began in March 1983, based on information by a now discredited informant who alleged that the group was supplying financial support to two terrorist organizations and preparing terrorist activities in the United States.

The probe was closed in June 1985 after the Justice Department concluded the group was involved in political activities and was not involved in terrorism.

While the FBI earned the criticism, it also deserves some sympathy. The agency has a difficult job trying to protect the U.S. against terrorism. It must investigate many organizations with philosophical or actual links with other countries. Doing that job while not violating political rights is not easy. Agents must walk a very thin line between protecting the country and safeguarding people's constitutional rights.

But it is gratifying that top FBI officials continue to be aware of this sensitive balancing act and have the courage to take action when agents stray from the guidelines.