That's not quite true, Voyager doors can be locked from the inside, and usually are. The problem is we can't run with passengers in a vehicle with a certain amount of locked doors for obvious safety reasons. I don't know the method of working at the place you're discussing, but at other stations if we call with a Voyager which is too long the rear vehicle(s) has to be locked out of use for the entire journey until we are past the last restrictive station, or passengers moved then all the doors locked and the vehicle placed out of use before calling at the station.
Meridians, which have an excellent SDO setup, are far superior in this aspect.