The bikes are locked but not fastened to something. So the butlers simple lift the bikes, locked and all, and put them in the right place.

I would guess they use rear wheel sliding locks [1]. Here in Amsterdam they are really popular.

[1] http://austinontwowheels.org/2008/08/06/review-axa-defender-...

I was wondering that too. Perhaps the low crime rate means people don't lock their bikes to solid objects?