How could this be true - even if the AJMC study is absolutely correct? During the study period (1999-2005), laparoscopic bariatric surgery was restricted to the highest risk patients, and done in a relatively small number of facilities by highly skilled high-volume surgeons. If the procedure is used more widely, it's likely to be employed in patients with less of a "pay off." It's also likely to be done by more surgeons with less expertise, at centers that don't have the same emphasis on multi-disciplinary programs.
Further, in an accompanying editorial another healthcare economist points out that the savings in this study actually represent increase of cost of controls - those who had bariatric surgery didn't have lower post-operative costs!
That's not to say that health plans shouldn't provide coverage for this procedure. In fact, as the editorialist comments, few covered medical procedures pay for themselves.