Sportbikes get crashed a lot, and they get stolen a lot (likely to replace all the parts on the ones that get crashed), so their insurance premiums are sky-high. The rental companies have to pass that along to their customers.

That said, here's a sport bike rental place. Don't have any personal experience with them, but they say they rent in the O.C. (They were the first hit under a search for "sport bike rental Orange County" )

Good luck!