I'm slightly confused or perplexed by your rhetorical title but I won't let that alone stop me from making a comment........H-U-G-E assumptions following.......
I'm assuming the van was unlocked even though it was most likely highly evident people were inside---probably why the robber approached, thinking the occupants wouldn't resist him. An unlocked door is NOT an invitation no de facto
permission for unwanted or unauthorized other people to enter a vehicle or other place. I don't believe a statue exists saying unencumbered access to another's property or residence/vehicle forbids anyone from protecting said property or residence/vehicle.
In fact I can't think of a more complete demonstration or example of the castle doctrine than this story.
Someone opening one of my doors locked or not and dares cross the threshold runs an extremely high risk of defensive action being immediately brought to bear with full intention to stop the intrusion completely.
This linked story to my mind is simply one for the good guy who took action to defend his family and property, zero for the bad guys--case closed.
Sorry if I took the wrong path here AT---just my reaction from having dealt with this sort of thing my own self.