Why can't they use the roads? Because the relevant authorities, here and in Auckland, not only constructs roads with no shoulders or cycle lanes, but in the case of the waterfront, creates cycle paths that have no clear demarcation that keeps pedestrians off. From the Railway Station to Oriental Parade there is no shoulder or lane, and after Oriental Parade there is a shared pedestrian cyclepath (the pedestrians walk all over the nominal cycle part) and a bunch of angle parking where parked cars reverse into the zone where cyclists are.

The last time I read the Road Code cycling on the footpath was illegal, but in Auckland and in Wellington this seems to be becoming unofficial policy. Roads are only for cars, you know.

I agree that normal cycling speed among pedestrians is antisocial, but the built environment definitely encourages it.