It’s true that EU regulation on this topic was harmonized in 2011. However, the harmonization actually loosened the Swedish requirements in several ways, particularly with regard to rear lights, which previously were required to be always on.
I bought a car (Citroën) a few years ago, and told the dealer I wanted it so the rear lights would always be on. They configured whatever doohickey is responsible for that, and made sure to charge me for it. But now the rear lights are always on, like they're supposed to, and I feel a little safer.
I think it's super dumb that it's not a requirement any more, and I don't see any reason for the change.
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).
