I think it has to do with the reason or agenda for the attacks. I don`t know why he did this, but I guess to classify it as a terrorist attack it would come down to whether he was just a psycho serial killer getting his rocks off or somebody who wanted to kill Americans in the name of Islam (or to impress democrats, same thing) or whatever.
It wasn't until well after they were caught that Malvo made his jailhouse doodlings. I think that had more to do with the fact he wanted to believe he was a part of something larger and more important, rather than just being a criminal. The fact they were converted muslims likely fed into their already disgruntled and murderous personalities, but no evidence points to it being the main reason for their crimes.
The snipings and panic is caused was certainly terrorizing. However, they were no more terrorists than University of Texas sniper Charles Whitman was.