While there's no legal obligation to know them, there's no legal obligation for you to be let through the border - for non-citizens crossing USA border, the explicit legal presumption(starting from [1]) is that you're not fulfilling the conditions of the visa and you have to convince the immigration officer that you are entitled to entry. If they want to check certain information that is not available for you (no matter if the reason is good or not) and they are not convinced otherwise that you don't have any risks, then there's no "beyond reasonable doubt" like in criminal proceedings - if they are not convinced that you're not going to e.g. work in USA, then they have not only the right, but a duty to deny you entry.

[1] (b) Every alien 10/ (other than a nonimmigrant described in subparagraph (L) or (V) of section 101(a)(15), and other than a nonimmigrant described in any provision of section 101(a)(15)(H)(i) except subclause (b1) of such section) shall be presumed to be an immigrant until he establishes to the satisfaction of the consular officer, at the time of application for a visa, and the immigration officers, at the time of application for admission, that he is entitled to a nonimmigrant status under section 101(a)(15) .