I'm fairly sure that we (legally)
have to register to vote in the UK, if we are UK citizens, and that some foreign nationals
can register to vote. I wish we had the Australian system of being required to vote combined with the US system of being able to vote for "None of the Above", but we don't, so I just comply with the law as it is. [I vote because I feel that those who
don't make the effort shouldn't bleat about how we are governed. If they are silent in their complaints/non-voting, I don't make it my business.]
So, to me, it seems like asking a person if their car is insured [legally, if it's on the road, it is supposed to be insured by law]; it feels a little "judgy" rather than intrusive, and the people most needing to be asked [by the police, not me!]
may be the people most likely to say "yes" when what they mean is "no". I'm just guessing on that, but I watch enough court TV to know that lying is a popular alternative to complying with the law. I would regard the (hypthetical) question about flossing as (slightly) more intrusive, because there's no legal requirement for a person to take care of their dental health.
I wouldn't ask the question, because I wouldn't want to be seen to be starting a discussion about politics. [I might
engage in one (though mostly now I just keep my big mouth zipped; I want to keep my friends of all political persuasions), but I wouldn't start one.]