It is arguable that they do have a moral obligation. Oh, not a strong one, but in any quasi-representative government, or other form of collective punishment (I kid but only just), making a choice has consequences for everyone. Which, they and the rest of us, are finding out.
NOTE, this is a sliding scale, there is NO moral equivalency between voting for Trump and NOT voting. One is an active effort towards converting the USA to one of the many flavors of authoritarianism, and the other is a passive abrogation of responsibility. One is a mortal sin as it were, and the other venial. But it still has it’s consequences.