Like many posters have said here, in today's "PC" world, "Assault" can literally be someone looking at you with a funny eye.
Maybe the guy does have a temper, which could have contributed to his great acting in Batman Begins (I haven't seen The Dark Knight yet)
I dunno. You've got be a shitty person to get in a verbal argument with your son/brother -even estranged- and even if it involves some brushing or light pushing to call the police on him a day or two later. This incident would seem to speak more of his sister and mother than Bale himself.