Had INS management strictly enforced federal law, it would have deported Atta. Instead officers decided to waive the requirement that he have a proper visa. Further, the agency didn't even make Atta fill out necessary forms or pay a $170 fine, as required under law.
INS Supervisory Inspectors have broad powers of discretion.
Inspectors are encouraged/instructed to use the least amount of authority/power as possible.
This is how I heard it: When Atta entered, he stated on primary that he was a visitor, and was thinking of taking flight lessons. This statement was alone might not get you sent to secondary. The officer on primary, may inform the subject that flight lessons would be a violation of his visitor visa and let him know that he would be subject to deportation, if he was caught by INS.
Depending how the subject acts on primary, depends on if he is referred to secondary.
In secondary, the same thing would be stated, and if the subject states that he still wants to attend flight school, he can be offered a Waiver of Documents, which cost $170.
Atta most likely stated that he would not attend flight school and would just visit friends or family, and was then let in on his B2 visa.
Atta was not dumb and was probably convincing, and as such released.
Depending on the Inspectors, he could have been refused entry and removed, forced to pay $170 or be removed or convice the inspectors that he would not attend flight school, and be aloud to enter.
Atta Skillfully Evaded Authorities in the U.S.
"Published reports that Immigration officers improperly allowed terrorist Mohamed Atta to enter the United States at Miami International Airport are factually incorrect," the INS said in a statement yesterday. The agency was reacting to a report in the Miami Herald that the INS had missed a chance to stop the 33-year-old Egyptian before last month's hijackings.