Both enabling and disabling incur a cost (because they delete the data, but then have to recreate it), but they wouldn't want to punish you for enabling it so it makes sense that the limitation is on the disabling side.
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.
They can put a giant warming in front of the last "off" click or whatever, not it needs to be there.
This reeks of MS thinking very MS-centric and hoping they fortunately retain some users.