I agree that it isn't due to being a shaft drive. If it were, I think it would affect all bikes about the same amount. Mine will drift left, but not even noticeable with one hand (or even a finger) on the bars. Some bikes are really bad, others almost nothing, some don't drift at all. It must be something about assembly rather than a design flaw - unless the flaw in the design is that it can be assembled out of spec somehow.....
My 1600 tracks straight, but the left pull has been reported enough times to be considered a legitimate concern. I think, however, it is a geometry problem, not one of physics.