See the various reviews about them coming apart and cutting people in the mouth. Being able to scrape off the logo with a fingernail is the usual giveaway.
My impression here is that some people are getting fakes, possibly because they are buying from marketplace sellers. The reviews don't distinguish but the number of positive reviews suggests this?
That said, one has to ask why Oral B, and others, are allowing their brands to be tarnished in this way. I'm assuming there are many more dissatisfied customers who don't take the trouble to leave a review?
If you expand the replies to most of the unfavorable reviews, it becomes clear that the fakes are from marketplace sellers, and not the main listing. As you say though, Amazon and Oral B should be doing something about it.
http://www.amazon.co.uk/Oral-B-TriZone-Electric-Toothbrush-R...
http://www.amazon.co.uk/Oral-B-Precision-Clean-Toothbrush-Re...
See the various reviews about them coming apart and cutting people in the mouth. Being able to scrape off the logo with a fingernail is the usual giveaway.