noumpere posted:99% certain there's a FED case play to the effect that "if the batter has an opportunity to vacate the box, he must do so ". That's likely (but not always -- especially with a short backstop and a rebound) to be the case on a passed ball.
I think J/R would call this "willful indifference"
I realized I have an old (2012) casebook on this computer... It doesn't have a specific ruling in that version, but this is the rule:
(7.3.5)
ART. 5 . . . Interfere with the catcher’s fielding or throwing by:
c. making any other movement, including follow-through interference, which hinders actions at home plate or the catcher’s attempt to play on a runner, or
d. failing to make a reasonable effort to vacate a congested area when there is a throw to home plate and there is time for the batter to move away.
It should not be construed to mean that the batter has to make an active effort to get out of the way except in cases where there is a play at the plate. On throws to other bags he can remain in the box as long as he doesn't make a movement that hinders the catcher from making his throw.