Jamie
You are correct in what you say but some on this site are advocating all sorts of mods. without getting approval. These mods range from dark sticky back plastic window tints, different type of light bulbs(internal and external) and of course mods. to audio systems.
Without assurance from Motability that these adaptations are OK and covered by insurance then any damage to the car caused by fire or an accident could result in the car being written off at their expense and the ghastly thought of no third party insurance.
If people really want to modify a lease car then have the courage to ask first.