The target audience may be slightly different. In London, the people I see wearing helmets while cycling tend to be that particular young male demographic, whereas people partaking in the bicycle sharing program tend to be commuters. As such, the speed at which both travel, and the roads that they take, tend to be completely different. If you're doing a high-speed bicycle-only commute into Central London then I'd say wear a helmet, but if you're doing a short-distance trip on a shared bike, there's really no need since you'll be travelling slower anyway.