Respecting “fiduciary duty” in Wikipedia I found:
A fiduciary duty is the highest standard of care imposed at either equity or law. A fiduciary is expected to be extremely loyal to the person to whom they owe the duty (the “principal”): they must not put their personal interests before the duty, and must not profit from their position as a fiduciary, unless the principal consents. The fiduciary relationship is highlighted by good faith, loyalty and trust, and the word itself originally comes from the Latin fides, meaning faith, and fiducia.
When a fiduciary duty is imposed, equity requires a stricter standard of behavior than the comparable tortious duty of care at common law. It is said the fiduciary has a duty not to be in a situation where personal interests and fiduciary duty conflict, a duty not to be in a situation where their fiduciary duty conflicts with another fiduciary duty, and a duty not to profit from their fiduciary position without express knowledge and consent. A fiduciary cannot have a conflict of interest. It has been said that fiduciaries must conduct themselves “at a level higher than that trodden by the crowd” and that “[t]he distinguishing or overriding duty of a fiduciary is the obligation of undivided loyalty.”
Secret commissions, or bribes, also come under the no profit rule. The bribe shall be held in constructive trust for the principal. The person who made the bribe cannot recover it, since they have committed a crime. Similarly, the fiduciary, who received the bribe, has committed a crime. Fiduciary duties are an aspect of equity and, in accordance with the equitable principles, or maxims, equity serves those with clean hands. Therefore, the bribe is held on constructive trust for the principal, the only innocent party.
So, if an IRS agent has taken any kind of commission for the conduct of his activities he has breached his fiduciary duty. Maybe that is why the try to keep it a secret.