It's harder to find a reasonable use case to constantly opt-in and opt-out, incurring server side costs. Generally you either want it on or want it off. They do limit the cost of disabling it some, because they cache that data for 30 days, but that still means someone could toggle it ~11 times a year and incur those costs.

I don't know what they're seeing from their side, but I'm sure they have some customers that have truly massive photo collections. It wouldn't surprise me if they have multiple customers with over 40TB of photos in OneDrive.