Just so you're aware, you already get background playback on mobile devices if you pay for YouTube Red.
But just the gall:
1. Get FREE creative work from musicians, film types, tv types, and tons of bright friendly ordinary people
2. Monetize the hell out of it (fine)
3. Allow a sewer community of racists bullies and other terrible people to develop in comments section (negligent but ok it's gotten a little better)
4. Make it hard for visitors to extract original media even though you're purporting to be a hosting service (lame but I can live with it)
5. Cripple playback in basic ways for petty reasons (you're starting to really suck)
6. Try and CHARGE people to undo 5 (Gah!!!!! Die!!!)
I pay for a lot of things. I pay Hulu extra for the ad free version. I pay HBO for HBO Now even though I have a pirated Go login. I buy tv shows on iTunes. I'll even buy movies off friggin' Amazon if it means not pirating them. But I will never pay Google to achieve BASIC playback capabilities on other people's content just on PRINCIPLE. Never. Greedy greedy little leeches. YouTube is a great thing but the people who run it have turned Bad.
Most of my favorite youtubers have been demonetized, and had to change their content if they wanted to survive. The Alternative History channel was gaslighted pretty bad and their G-rated animated mini-documentary videos got flagged for hate speech(?), while Logan Paul made tens of thousands of dollars from YouTube monetizing his disrespecting a Japanese corpse.
I suspect they could theoretically allow background play for other content but that would be confusing to users.
Even when I've used YouTube for music, the ads are always pre-roll. If I watch the pre-roll ad, why not let me background at that point?
Google could allow backgrounding for content with no ads. They could allow backgrounding of non musical content if you've watched the ad (they can detect if it's music and what song it is, that's how they pay royalties to labels on songs uploaded by random people). They could allow backgrounding of musical content if you've watched the ad.
YouTube doesn't attempt to handle any of these scenarios. Handling these scenarios would involve an investment of software and possibly legal resources — only to improve the experience of millions and millions of users, and to conserve tons of energy, without making Google any money, and even potentially reducing revenue to YouTube Red (or Premium or whatever it is next week). Why would a company in a monopoly position do that? I get WHY Google won't do this. It's not about licensing, it's about money and motivations. But I'm not going to pay into it.
Luckily, you can thwart it by: 1) Requesting the desktop version of a YouTube video 2) Replacing youtube.com with hooktube.com in the address bar
Perhaps soon Google will disable the use of labels in GMail and then charge you for "GMail Red" to get access to them again.
Shitty move on their part.