There must be some reason. I've seen people been asked to switched off phones whilst going out to the aircraft so it is policed by non airline employees working inside of the airport so it is policed somewhat.
Other than the fire risk there is also the potential safety hazards associated with an apron and talking or texting on a phone could perhaps reduce a persons situational awareness. I'd be interested to know why. I'd out money on it NOT being to do with speeding up turnaround times.
I can't imagine them making it up. If it was policy in order to speed up turnaround times then they would tell customers in the same way that they used to explain how it was the crews responsibility to clean the aircraft and there is a short turnaround so please put all rubbish in the bags provided.
EZY allow PEDS onboard when ever the doors are open. So if people were going to waste time it would be rummaging through their bags finding said devices and switching them on. It is only the portion of walking on the apron to the terminal where they are not permitted.