Hi, cobun.

"Practice" isn't limited to medicine. It is the common term used as well for active lawyers. The seventh definition in the Webster's dictionary for "practice" is "the business of a professional person," and the sixtheenth is "to pursue a profession, esp. law or medicine". I'd hazard a guess that this use derives from the third definition, "a repeated performance or systematic exercise for the purpose of acquiring proficiency", through the fourth definition, "condition arrived at by experience or exercise".