Mark Kelly explains
The local paper, very surprisingly, carried the story (I can't get the link to work for some reason) and gives his explanation: he bought the AR-15 to investigate how easy it was to buy.
C'mon, admit it. You bought it because it was a fun gun.
UPDATE: Let's not be too hard on the fellow. This may be a purchase he wanted to keep secret from his Mrs.! Married gunnies face that option from time to time.