[Adam]

I'm going to add my two bits here.
The ref you talked about in the original post was right. It's a judgment call. You're wrong in saying it's not open to interpretation. Look at rule 4-27; it clearly states that foul calls are jugdment calls. If a player pushes another one, and that other player later travels, it doesn't mean the relationship between the two events is causal. That's for the ref the determine.

If the ref thinks the push did not cause the travel (maybe he thought she was off balance before or maybe he thought she gathered herself first), then he's going to call the travel. If the ref thinks the push caused the travel, he'll call the push.

The fact is, it's a judgment call for which the responsibility is on two people in the gym.
Furthermore, most middle school refs are struggling with how to apply these things. He may have initially thought the contact was incidental and didn't want to come in with the late foul. I've called the late foul on this play; but it takes some experience to do that.