The rule is more about ensuring that we have our passenger's attention during the two stages of fought where things are most likely to go wrong, as opposed to concerns over electronic interference.
It's just easier to say that it's to avoid system problems.
Still, people don't listen. They never will. The Air Nav Order needs to be enforced more strictly. If you stand up on arrival before the seatbelt sign is off, I think you should be fined.