You seem to be diving directly to the low-level settings. I don't know why those would be locked, but the telemetry checkboxes in the UI work just fine, and are

You seem to be diving directly to the low-level settings. I don't know why those would be locked, but the telemetry checkboxes in the UI work just fine, and aren't disabled. And once disabled, they'll stay disabled. Firefox is never going to prevent people from disabling telemetry.