If AQ wholly intends on bombing the U.S. again, via planes, then why are they ALSO trying to get in through Canada? What's the point if they don't have the bomb-making materials or ability to obtain same once they're in the U.S.
They wouldn't be coming through the northern borders, either, if there wasn't some nefarious reason for trying to come into the U.S. other than via plane.
Think about it....Do you think there where ALWAYS coming into the US via US only flights?
They weren't. It makes a certain amount of sense to try to vary the ways you get to a certain place.