People in jobs in the Secret Service are no different than people in the public sector. Rules that "normal" employees need to follow are no less valid to "special" employees in the Secret Service or other government-based positions. There needs to be, at the very least, some sort of system of reprimand for the first occurrence and losing jobs after repeat occurrences.
Yes, Secret Service employees should be fired for violating alcohol restrictions. The individuals who are selected for these positions are supposed to be the cream of the crop. These individuals should realize they are role models and are looked up to simply because of the long standing reputation of honor and integrity in this field.
The secret service needs to remain secret, and they need to keep a lot of secrets in order to protect the public. The effects of alcohol on the body have proven that it is hard to be capable of holding ones tongue. Secrets would come out and a lot of them could be dangerous. Other people can not drink where they are working, the secret service should not be immune to this standard either.
Secret Service Agents should be fired for violating alcohol restrictions, especially if they are on duty. These restrictions are made known ahead of time and if they can't abide by them, then they shouldn't have the job. There are plenty of people who want to work honestly and want to follow directions.