This is much to do with honour, or acting honourably.
Full disclosure relates to specific arrangements under which a person is expected to disclose everything about or under a particular heading put to them.
Now some people will look at the wording of the disclosure and seek to find any way that they can provide 'full' disclosure without actually giving the fullest disclosure that they could. They seek to circumvent disclosing all.
Therefore, when asked to reveal anything in the spirit of full disclosure, although there may not be any legal provision to do so, the person is asked to be honest and honourable.