It seems to me that there is a bunch technology and R&D that takes place in WSBK, as opposed to AMA. AMA rules limitations are pretty severe whereas WSBK allow for a lot more freedom. One would normally think that this would make the WSBK bikes much faster, but not really, as new technology often has unseen consequences. The same Crash.net article that talked about Mladin's lap times also talked about traction control on Corser's WSBK bike which was not on Mladin's AMA bike. Big difference in performance and rideability and the way you need to ride them.
Take tires, for example. Mladin's been running with Dunlops for more then 7 years and knows them inside out. Pirelli's only been the spec tire for two full years, and Suzuki has largely been a non-player in WSBK until last year. That's a steep learning curve and a lot of variables. Given that Mladin is only .59s faster, based on fastest laps, that's not too shabby.
I think Mladin stays in AMA because he would rather be the big fish in a little pond. Remember that he used to run for Cagiva in the 500cc class back in 93-94 and he was terrible and in the prime of his life. Maybe his style was not fully tuned, but when you get smoked by the likes of Darryl Beattie, you've got issues.
He could jump to WSBK or MotoGP at the drop of a hat. Suzuki would build him a bike if he wanted to fill in or just do a one-off ride but he would probably not want to take the chance of looking like an idiot. Yes, Hodgson has had trouble in AMA. But I chalk it up to the track and track surfaces. After three or four years running on the best tracks in the world, Infineon and Mid-Ohio probably scared the crap out of him.