TRACON Controller described a loss of separation between an IFR aircraft cleared for a Visual Approach and IFR traffic inbound to an adjacent airport when the Visual Approach aircraft made an unexpected turn to loose altitude.

Narrative

A King Air was set up for a right traffic downwind into the airport. I cleared the pilot for the Visual Approach to Runway 28R. The pilot continued eastbound to adjust the altitude/position in relationship to the airport. Instead of making a right base he did a left 270 to base. This maneuver took the aircraft back towards the neighboring airports final approach course (the other airport is only 4 miles north of the King Air's destination airport). As soon as I noticed this maneuver; I instructed the pilot to immediately turn south for traffic. A F18 was on Visual Approach to the other airport and was already talking to that airport's Tower. The pilot's unexpected left turn to final resulted in a loss of standard IFR separation; after reviewing the event the reported closest proximity of 2.79 miles. I initially reported the event to management because I thought it was surely a pilot deviation. I was advised that the pilot is authorized to turn AWAY from the airport on a Visual Approach. I don't know if they are correct. Every Controller I presented this situation to have had the same response; we all believe that the pilot was in error. At minimum it shows a lack of common sense on the pilot's part. I recommend a change to AIM and or 7110.65 to show 'Pilots on right traffic Visual Approaches shall make right turns and pilots on left traffic Visual Approaches shall make left turns.'