I wondered the same thing myself.
If the Secret Service agent took an oath to the Constitution, to defend it from enemies foreign and DOMESTIC, maybe he's just following through on his oath.
Well he didn’t sign The Official Secrets Act like in England. Are agents required to sign Non Disclosure Agreements or something to that effect?
This Former Agent is writing about what when on almost 20 years ago. I think that is a long enough time period for him to be able to say whatever he wants.
The SS agent may be less than honorable, but when someone acts terribly in private, he is vulnerable to people that stand to gain by exposing or blackmailing them.