Is the batter-runner out?

[M&M Guy]

I'm writing this as a fanboy with a question:

ASA slow-pitch rules - batter hits a popup to short right field. He starts to jog towards first, but it looks like an easy out, so he turns and starts walking back to the plate. The right fielder mis-judges it, and the popup falls in. The batter, meanwhile turns and sees the ball drop in, and runs towards first and makes it without a play.

The umpire calls the batter out for "retreating towards the plate". I thought this only came into play if a play is being made on the runner? Or is it in all cases, play on the runner or not? What's the appropriate rule?

Thanks.