Heathrow incident
TC I don't think that is the reason. I left Boscombe on posting to Heathrow in Nov' 86 just after the incident, and the only person stopping visitors plugging in was the manager of the 'affected' watch, dear old Tom Toy.
I joined his watch in early '89 and he was still adamant that no-one could have a visitor plugged in with them. After he left to go to TC in early '91 our watch allowed visitors and I can recall several occasions when I had non-aviation people with me right up until my early retirement in 2000.
My son when he showed an interest in joining ATC in '95, plugged in with a mate on TC and then another in En-Route at LATCC, and he himself has offered to show people around until well into this century. It does seem to be a fairly recent blanket restriction.