Double White Centre Lines

I wonder how many of the great motoring public really know the meaning of the Double White Line in the middle of the road?

From the Highway Code (see http://www.direct.gov.uk/en/TravelAndTransport/Highwaycode/DG_070306 Rule 129, ) I quote:
" Double white lines where the line nearest you is solid. This means you MUST NOT cross or straddle it unless it is safe and you need to enter adjoining premises or a side road. You may cross the line if necessary, provided the road is clear, to pass a stationary vehicle, or overtake a pedal cycle, horse or road maintenance vehicle, if they are travelling at 10 mph (16 km/h) or less."

Now folks that's me - when I'm going down the hill at 25 ish on my bike, that means I am doing *more* than 10mph and thou shalt NOT overtake me. In fact most of the time - unless I'm struggling uphill - I'm doing more than 10mph and though shalt not overtake me.

Why do they do that?
(and don't get me started on Rule 163)