Why is this case being tried by court-martial?

This Tampa Bay Times report describes a U.S. Coast Guard court-martial. The main charge is assault on a woman in St. Petersburg, according to the Coast Guard. If the assault took place off base and (one assumes) the victim was a civilian, why is it being tried by court-martial rather than in the Florida civilian courts? (Yes, it's constitutional under Solorio v. Unites States, but is it sound?) Did the "downtown" authorities refuse to prosecute? If so, does that justify use of a military court?