Suppose Microsoft adds this capability to Windows, and you edit the registry to disable it. How is that any different?

By allowing the end user to change it instead of locking it down, they are not making a good faith effort to comply, and they lose their liability protection. To maintain their immunity, at the very least they will need to prohibit Californians from disabling the feature.

Canonical is prohibited from adding comparable terms.

I can see the argument for something like iOS.

How is iOS any different from Windows here?

Let's say you own a computer store in California, you sell Windows laptops, and you setup your preinstalled Windows image with the registry edit made, because customers don't like the silly age prompt. How are you not the OS Provider?

Again, to maintain their immunity under this law, they would have to prohibit me from doing this in their licensing agreement. My violation is what protects Microsoft. I would, indeed, be the OS provider in that scenario.

But in the scenario you describe, I'm not the end user.

Neither Canonical nor I can include the same restrictive terms in our OS offerings. We can simply inform our users that the OS is not California compliant. Our users become their own OS Providers as soon as they decide to use them in California.