A question can have presuppositions even if it's not rhetorical. If I ask you whether you have stopped beating your wife, I'm implicitly claiming you have a wife and were beating her at some point, even if I'm genuinely curious as to whether or not you're still doing so.

QY is implicitly saying that brainoil must think voting is valuable relative to the other things ey could be doing, with which I either agree or ADBOC depending on what exactly is meant by "valuable".