It must be some sort of standard procedure to not allow the punters to use mobile phones or cameras on the apron using refuelling as the justification. Certainly nothing in the current transport security regs preclude anyone from taking photos airside. However the airport operator can decide who may and under what conditions can take photographs. So rather than use any common sense to allow an acitivity to occur when it is safe, a blanket ban is much easier to enforce.
The exclusion for radios, mobile phones etc around the fuel truck and delivery point on the aircraft from memory is 15m so if you are outside that arc, I fail to see what the problem is. No shortage of ramp rats running around with radios which I'll bet my left one are NOT certified as intrinsically safe.