Do the agents take an oath not to say anything? Are they bound by some law? Or, is it just a custom?
IIRC they take an oath that includes a statement that they will not disparage the office holder. I could be mistaken.
I’ve had the opportunity to spend some time with a Secret Service Agent. He would not criticize anybody who had been under their protection, including John Kerry. After some serious prompting he did agree that Kerry “acted abrasive” towards his security detail during the last election.