This is silly. If passed, your boss may not be able to explicitly punish you for not responding to after-hours email, but he can certainly make decisions about

This is silly. If passed, your boss may not be able to explicitly punish you for not responding to after-hours email, but he can certainly make decisions about promotions and discretionary bonuses based on whether you responded to after-hours emails, without saying why. All this does is encourage people to be devious.