The likely scenario is that information distributed throughout the agency suggests that they do not spy on US citizens. Basic training is done for all employees where they are taught not to spy on US citizens.

However, NSA also conducts clandestine operations. Employees selected for those operations are probably recommended based on their willingness and trustworthiness when it comes to spying on US citizens and effectively breaking the law.

Alternatively, as has been suggested recently in numerous articles, FBI probably works hand-in-hand with NSA to "legally" allocate information required for datamining. To the employees of the clandestine operations, the information obtained is technically "legal", so perhaps that serves to settle the nerves of the employees involved.