is this a balk ?

[BUC]

Scenario: In a AAU 11-u baseball game, runners on first and third, f-1 comes set, then steps of the back of rubber with pivot foot, fakes a throw to first, then turns and throws to third. The umpire called it a balk saying fed rules prohibit any fake throws to first. I thought once f-1 stepped off the back of the rubber he could do anything/fake anywhere ???