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".