Niknac use your head, crews mix with inbound/outbound pax all the time - on the aeroplane.
Outbound crews (who have mixed with outbound pax) need to be able to talk to inbound crews (who have mixed with inbound pax) on the a/c to hand it over etc etc.
Crews are trusted with millions of pounds worth of equipment, duty free bars, yes, even the crash axe etc etc etc.
To suggest this is anything other than empire building by jobsworths is nonsense.