You're simply not using the word 'fiduciary' correctly. You seem to have expanded it to mean any sort of legal or ethical obligation with respect to markets, and that's not what it means.
The right against self-incrimination is limited in civil trials. Additionally, unlike criminal trials where a jury will be instructed that a defendant's refusal to answer cannot be construed as evidence, there is no such instruction in civil trials, and it is common to argue that a refusal to answer indicates hiding something.
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