I don't know why some bikers do this, but I know you're not the only one that notices this practice. An article in The Citizen yesterday (looked but couldn't find it) had a statement at the end of it that the Bike Week organizers and Laconia PD strongly discourage what my buddy refers to as "the centerliners".
Found the link:
http://www.citizen.com/apps/pbcs.dll...WS02/706219919
This is the statement made in the article:
— With that said, motorcyclists really need to heed the advice of city and rally officials and stay off the center line. I can't tell you how many times I saw bikes riding dangerously close to oncoming traffic. I almost wore a few as a hood ornament myself and it is downright scary for those of us in cars.