I think the government's position may be that travelers do have a right under the fifth amendment to decline to answer these questions if they believe that answ

I think the government's position may be that travelers do have a right under the fifth amendment to decline to answer these questions if they believe that answering could create a risk of criminal prosecution, but that the government can still use that refusal to answer against the traveler (for example, denying a visa or denying entry to the country), because the government contends that immigration procedures are "administrative" and not "criminal", and being denied entry is merely an administrative decision and not imposition of a criminal punishment. In general the government has only accepted the idea that refusal to answer questions can't be held against someone in the context of a criminal trial. For example you can certainly take the fifth in a deposition or examination in a civil case, ideally on an attorney's advice, but this decision can be held against you for civil purposes (for instance, the judge can instruct the civil jury that it can consider why the witness didn't want to answer the question). The government's position is most likely that the immigration proceedings have an analogous status where you may be allowed to decline to answer, but an adverse inference can still be drawn from your refusal (like making an assumption that you had something to hide).