Earlier quoted context omitted.
Mobile providers are a different beast because there’s a limit on radio spectrum. Throttling is necessary.
Let's take the null hypothesis that these incentives push all video streaming to Apple+. The total bandwidth hasn't changed, all that's happened is that is it's only streamed over Apple+ now. How does this help with their spectrum constraints?
If a music streaming service participates to label its traffic, that data is zero rated for eligible users.
If a video streaming service participates to label its traffic and limit bandwidth (advertising says 480p, but I think it's really a bitrate cap), then data is zero rated for eligible users. On plans with a high speed data cap, users can opt out of the bandwidth limiting and get potentially higher quality streaming (depending on network conditions) but the data usage will be charged against the cap.
There's no charge for providers to participate.
I don't know how zero-rating music streaming helps with spectrum; that's probably more of an inducement to use data, but music streaming is low bandwidth, and maybe there's some magic to accumulate those packets and send as a batch rather than one at a time and therefore save some framing bandwidth.
For video it's clearly a bandwidth thing. If you can reduce to the target, that's good for their spectrum management, so you get a carrot. Also, it encourages bandwidth sensitive streaming (which many services were already doing), so that if the conditions are poor for a particular user/cell, video bandwidth will be responsive and reduce automatically.