The danger from knives is obvious to most children, and a cut probably isn't going to seriously injure them. Knives are not marketed as toys, and have a use other than being played with. If a child is injured by a knife, you will know it quickly, and understand the extent of the problem. Half of the people in this thread are comparing it to swallowing a penny, which is so harmless that you could make it a hobby.

This is not health & safety gone mad.

The difference is solely intent of the product. If bleach was being sold as a toy then it would be banned; it's only allowed because its not a toy. Lawn darts are less dangerous than knives and they were banned because they are being sold as a toy.

There is a certain threshold of danger that we accept for useful products like knives and bleach; if handled properly then there is no problem. Young children aren't expected to handle them properly. Buckyballs have no practical use and are toys that are expected to be given to children, so a similar level of danger is unacceptable.