You may not expect a reply but there are a great many people who would feel pressure to reply to a senior staff member outside of work hours.
If someone wishes to volunteer to work 24/7, it isn't my job to stop them. It's insane of them, though, and I might contemplate firing someone who felt pressured to act/respond at 2am (for being dumb and unreasonable).
Just because you think there is no pressure to reply, doesn't mean there isn't a societal/workplace culture that pushes people to do so when their superiors ask for something. Same reason you can tell a co-work something is dumb, but you would say 'lets look it from another angle' to a manager or ceo.