First off, the movie is JB/GR with a very few elements of DK/GR in with it. JB had a chopper as a run-around bike, not a cafe-racer. The movie is set to look like Texas and choppers are a better choice than cafe-racers for highways. Plus choppers have a lot of options that cafe-racers do not have, examples are fork lengths, handle bar style, exaust style, to have sissy bars or not, etc. On cafe-racers you have to have a certain kind of handle bar design, seat design, exaust design, etc. Choppers look more bad-ass cruising the highways than cafe-racers and can shake the ground if you have the right exaust. Who doesn't want to take either bike to Bike Week or another show?
The movie is live action so it stands to reason that they want ALL the elements in the movie to look as REal as it would in real life. Computer animated characters do not mix well with live actors.
Enderwaves, you would probably like the bike at
http://www.countingdown.com/movies/15658/board?viewpost=3659850&folder=0.
Hunter Rider, where did you learn that Jesse James designed the chopper? That would be an enteresting read.
4CABGs, remember that Caretaker has at least one horse. There is also a slight resembleance to a horse skull with the skull on the hellcycle.
For the point I told of priar, (The front wheel on the bike is definitaly CG 'cause it is not touching the ground and the forks probably are not real or not that big of chain links, though there has been chain link forks) the flames are probably CG as is the reflections of the flames and the repeating chain links (because of the computer animated look to them).