I think the R & A/USGA statement is not as clear as first thought.
My interpretation is that it prohibits the use of a device that has a present capability of doing something non-conforming; eg. gauging or measuring other conditions that might affect play (e.g., wind speed, gradient, temperature). By that I mean the phone has apps installed at that time (therefore, a capability) that can perform the non-conforming functions (irrespective of whether they can be turned off), the phone must not be used during play. Contrast that to a situation of a smart phone which, has the ability to download apps which would make it non-conforming, but those apps have are not installed at the time of play. IMO the latter situation would be fine.
Given that golf is a largely self regulated game, I can see sense in the "it doesn't matter if you can turn them off" position.