In response to a post about the 1901 Royal Enfield, David Blasco wondered some time ago if the unusual position of the engine was in response to patent issues. What he is referring to is that when Enfield decided to lower the engine to the currently usual position in motorcycles, under the rider's legs, they confronted the fact that apparently motorcycle maker Werner had a patent on the positioning of the engine there. Enfield circumvented this by bending the frame.
It is interesting to speculate about these events. Could Enfield, as David suggests, have placed the engine high only to circumvent the patent? According to Peter Hartley the answer is negative. He claims in his book that motor-bicycles of the time were notorious for the slip-slide phenomenon in which the machine would slip sideways. My hunch is that the origin of the problem was that these were essentially flimsy bicycles with skinny tires fit with an engine that added considerable weight and traveling at much higher speeds than advisable on roads that today we would not see even in the Third World. It should be recalled that at the time in the UK most vehicles were animal drawn! However, Jules Gobiet, who at the time was working for the Enfield company having arrived from France (which at the time had a much more advanced auto industry than the UK due to the absurd Motoring Act of 1865 that limited speed to 4mph) had different ideas. Incidentally, he coined the motto "Made like a gun". He noted that at the time many motor bicycles had the engine up high like the Enfield, but they drove the front wheel. Whereas pedal bicycles drove the rear wheel and did not have slip-slide! Bingo! Keep the engine up high and drive the rear wheel was his answer. Of course, this did not work, so for the 1903 model year, Enfield decided to lower the engine to the now usual position in motorcycles. There they ran into the fact that Werner had a patent on the frame using the engine as a stressed element (as in the modern Bullet). Enfield circuvmented this by bending the frame under the engine and not having the engine as the stressed element.
Hartley does not quote any sources for all this, so we can only take him at his word. But I'd say that the explanation David suggests makes somewhat more sense to me.
Triumph introduced in 1902 a motor bicycle with an engine that was a stressed member of the frame. Why didn't they face the Werner patent issue? Perhaps the fact that Enfield had bypassed it so easily convinced other brands to simply ignore it? If that is case, Enfield's role in the origins of motorcycling is even more remarkable.