I suspect that this pressure over helmet wearing comes from a number of different sources.
We've had quite a few people say that they feel safer wearing a helmet due to the fear over high traffic densities. More have said that they wear one to allay the fears of their loved ones about cycling on busy roads. This is not something that can be condemned in any way: if a helmet means that someone gets on a bike rather than in a car, that's unreservedly a good thing. Of course, that cycling is not at all dangerous is a topic that has been done to death many times before and will doubtless come up again. But the fact remains that, rightly or wrongly, cycling is seen as a risky occupation.
It therefore seems logical to reduce that perceived risk by wearing a helmet. It seems like common sense, doesn't it? It's a logical step that's reinforced by the unceasing campaigning by Headway, the BMA and others over helmet use. They're doctors, so surely they must know... But this is highly one sided: the actual science is very much more ambivalent over the benefits of helmets in cycling: this is a message that simply does not get through. When bold statements from authority figures supporting helmet use go unchallenged, it is no wonder that the idea that helmet = safe is so widespread. There simply is no debate whatsoever in public circles over whether helmets are beneficial, specialist interest sites like this excepted. (And
@Eddy if you read
@User's link, you'll find that it refers to actual research papers. They are an objective report of the science done, they have to be otherwise they are unlikely to be published - and any biases in the methodology or interpretations are deeply damaging to the authors. Most scientists are extremely keen to avoid such controversy, if they wish to keep their positions).
There is also the fact that, unlike Europe, few UK cyclists are utility cyclists. It is not regarded as a means of transport, but as a sport instead. Indeed, most club cyclists will be found to be clad in the full helmet and lycra regalia. Until recently, there have been very few utility cyclists so this image of the helmet and lycra clad sportsman has been the only available one to new cyclists. I find it interesting to find that sports cyclists on the continent follow the same pattern. Since the perception of cycling is derived from sport, it is little wonder that most new cyclists don a helmet - and little wonder over the expectation that cyclists must wear helmets in society in general.
Anyway, enough of my babbling...