Also, smartphones have WiFi, so that's a way for the security-aware to control the device for typing your very important passwords into. Bring a Bluetooth keyboard and a phone mount with a built-in magnifying glass, and it's almost a laptop!
I think the target is more likely to be business users doing work during the flight?
In any case, broader laptop/tablet bans seem to be off the table for now.
The reason that phones are banned on planes is no longer due to concerns about interference with the aircraft (which have been, by and large, debunked), but rather because the terrestrial cell network is really not suited to supporting phones at altitude.
They allow you to completely disable flight mode, once you're at cruising altitude -- and therefore out of range of the terrestrial cell network.
(Whether your cell provider will actually allow your phone to associate with the in-flight microcell is a different issue...)
IIRC the "put your phone in airplane mode" rule is a FAA mandate. Airlines can't change it.