ANSWEARING, they do that because they are riding your Draft, it makes them go faster then their normal top speed.
[tried and tested]
i always give motorcyclists as much space as i can, Coz 9/10 times they're gonna end up passing me anyway, i'd just rather they do it with a Nod of acknowledgement.
50cc scooters dont count in my book, as all the 50cc people drive like IDIOTs, (they have no road sense)

Coz those bikes are CRAP,
i know Cos i've had one as a curtacy bike whilst my Motorcycle was being serviced.