Ben that's not the case. If you cannot provide concrete evidence that a cycle helmet does not work, to any effect, then you cannot make the claim. claims are substantiated with evidence.
you may argue that there is no reliable evidence to support the use of a helmet, but you would be wrong to conclude its inneffectiveness.
If you really think that, then you have no understanding of the scientific method. No-one has to prove that cycle helmets are ineffective as an argument for not wearing them. If people wish to promote helmet use it is up to them to provide the evidence that they are effective. That is how all public health interventions work.
And in any case I never said that cycle helmets do not work to any effect, just that any effect they have is not detectable, therefore it must be tiny.