Au contraire, screen-off playback is exactly the kind of feature that is and should be premium, because the value of advertising that can be sold in this situation is significantly reduced, hence it only needs to be available to premium users who are paying for access and thus aren't showed ads anyway.
"Premium" doesn't mean "fancy" or "hard to implement", it just means "We aren't making money giving this away for free, so you have to pay for it".
I completely disagree with your definition of premium. Something has to go beyond basic functionality to be premium, entirely separate of how it's being sold or not sold.
And someone with the screen off is using far less bandwidth. I doubt it's hard to make money off them, even with reduced ad revenue.
I read it as "this feature is premium specifically because it is not free." Businesses are under no obligation to offer basic services for free, even if they did in the past. If this is good for business is different, but YouTube must have had good reason to push all those features to paid. It doesn't matter how much bandwidth is being used, its to block people from using it as a streaming music for free.
In the concept of the freemium payment model, a premium feature is simply one that yo have to pay for. Maybe you should use a word like "advanced" or "deluxe" or something? Regardless, it's an orthogonal concern; it doesn't matter if it's the simplest damn feature in the world, it needs to be for paid users only. Call that what you will; many call it "premium".
"Premium" doesn't mean "fancy" or "hard to implement", it just means "We aren't making money giving this away for free, so you have to pay for it".