Dangerous and illegal? A tad strongly worded there considering we have have hardly any examples of the supposed danger (versus how many accidents happen on the road every day) and despite the police moaning at me for many other things over my cycling career, they never once in years did anything about me or the many others that cycle on the pavement.
Are you suggesting that putting yourself on top of a flimsy bicycle with no means of protection against collision or even a seatbelt, and then surrounding yourself with a load of heavy metal boxes going a lot faster than you and thus forced to get past you is anything other than dangerous?