I wouldn't imagine the power draw from always-on rear lights to be especially high compared to, you know, actually moving the vehicle, especially if those lights are LEDs. And the studies I'm finding measuring this exact concern don't seem to show particularly significant impacts on fuel efficiency / emissions for ICE vehicles (a couple percent, tops, and that's assuming small cars w/ tiny engines and some pretty comically inefficient DRL implementations - hardly relevant for creating a new standard that can mandate e.g. dedicated LED-based DRL systems).