I guess it's probably not that much about assault (it would be anyway only for "trusted" people like spouse of crew, member of the airline?), but could probably be more about dealing with flight procedures: how do you handle turbulences while in there, how do you handle an emergency, wake up before landing, ... Flight attendants will be busy in the main area in those situations and it would be difficult to take care about non-trained passengers in those spaces (small space, small stairs, ...). It seems for example that they have special breathing equipment there, maybe not like in the seated area: you'd have to know how to use it...