" the pictures show definite probable cause to approach both drivers as possible snipers given the most detailed descriptions of the suspect vans that have been made public." That's true. It's also true that these vans are common and it's more likely they have an innocent pulled over. So they pat him down and he stands aside as they check it out.
If they did have the killer, I'm sure he's not going to wait 'till the cops get the drop on him to go start a firefight. That starts before he looses the upper hand.
Ok a step at a time. Do you agree that these particular 2 vans closely meet the most detailed description of the van witnesses have placed at the scene of the home Depot shootings and you concede that approaching them more seriously is warranted? Do you also not see the possibility that there could be another person secreted in the back and that before they look inside they want the driver secured?