You don't know that it is BS. In fact, if one looks at the casualty statistics, it seems likely to be true, at least as far as deaths and serious brain injury are concerned.

Where helmets have been made compulsory, deaths and serious injury to cyclists have not declined pro rata to the number of cyclists. In Australia, for example, the number of miles cycled declined, but the number of cycling deaths per mile increased.

Therefore either helmets are not preventing any such deaths and injuries, or for some reason they are giving rise to at least as many as they are preventing. There are various theories as to hy this might be so. The helmet may, on impact, cause the head to rotate more than an unhelmeted head would, giving rise to trauma as the brain is bounced around inside the skull. Helmeted cyclists may actually be at greater risk of accident than the unhelmeted, possibly because of some risk compensation on their part, or on the part of drivers. Whatever the mechanisms, the data is clear. Wearing helmets has not made a contribution to the public health.

They're probably good at preventing superficial injuries in low-speed falls, which is pretty much what they are designed for. Personally i think this advantage is outweighed by the fact that their use seems to correlate with a higher incidence of more serious outcomes.