Title says everything. Basically, it would be more fun for players to join any store while begin robbed. Sometimes, it happens which a friend of yours robbing store, then you would like to join with him.
It's just for more fun and enjoyable to play.
I would rather think...
If you rob a store, other players should still be able to enter the shop but there won't be any services on/shop clerk to sell for you what you need.
Once it resets, it should go back to normal.
I will change it to allow you to enter a store while it is being robbed. It's not as if the door would be locked.