Armor stands are thought of as decoration, not entities. Designating them as entities is a technical distinction that should not interfere with the enjoyment of play. Therefore, a command meant to affect entities should not affect armor stands 

The problem I encountered is: I spent a lot of time arranging enchanted armor on a row of armor stands. I had need to clear all mobs from the world, so I used the 'kill @e' command. I was surprised and befuddled to find that all the armor stands "died".

It seems, to me, at least, that if armor stands are affected by that command, one might just as assume it would "kill" all item frames and campfires.