Entrance is not the main place where bees defend their home.
When a robber goes on combs, there it meet hundreds of workers which identify the wrong odor of bee.
If you look inside the hive, where is robbig on, main battle is on all combs.
So, to defend their hive, bees need a room, which is all occupyed. If there are empty places, where robbers may move freely, it makes robbing easy to start.
If robbers go out alive with honey load, they alarm more robber gang and pressure may rise too strong.