Oh it doesn't. It just proves that not everything is 'proveable' (yet).
I do not doubt some forms of dowsing and again I'll say what I've said before: Geofizz is just electronic dowsing. It just so happens that we do understand the principles behind that because we built the tools/instruments. Because we don't built the forked hazel twig we don't know its workings and won't accept them.
The problems with tests like Randi's etc is that they don't really test the claims. If a bloke says he can dowse for an underground spring in a field in Sligo he'll be tested to see if he can find a full glass of water in an office. Whatever force (if any) is causing the reaction the dowser experiences will not be present in different conditions. Until the bloke is tested in loads of fields in Sligo and fails we can not poo-poo his claims.