"Reasonable overtime" is only one example - there's a whole host of other really basic questions that you can't get simple, definitive answers to.
For example, can I employ someone on a 12 month contract? What about 24 months? 36 months? At some point, they become a full-time/ongoing employee and get the benefits of unfair dismissal/redundancy, which means the whole "fixed-term" thing becomes meaningless. The problem is, it's never clear at what point that actually occurs.
Gut feeling is that this is because all politicians know it's an absolute minefield, so none of them is willing to touch it. Better to leave it vague enough that no-one can accuse them of favouring business or employees, because no-one really has any clue what's going on. Kick the can down the road and let FWA deal with it.