Some people may want to totally mask what is out of frame.
Passepartout color is per theme. But it would make sense to have it per camera.
Because request was about necessity to change it per scene.
But being able to change it per view angle would be more pertinent, because colors visible in camera may be different according to objects of scene, dominant according to viewpoint.
What is making sense for colors of objects is also making sense for dimming effect of passepartout.
Some animated objects brighter from some point of view could require a stronger dimming according to point of view.
You don’t have to dig into hierarchy to modify all cameras passepartout settings at once. Just use shift G to select them all by type in Viewport. And then, a Copy to Selected is possible.
Only hidden cameras would not be affected. But if they are hidden, that means that you are not using them.
If you choose to create a custom property to drive alphas of cameras ; that is, at creation of camera, that you have to set/copy a driver. And then, you don’t care anymore.
Alpha of passepartout being just one property value, that should not be complicated to write a script, an addon that changes it for all cameras of scene.