I'l preface this by saying that I don't think any criminal charges should have been taken against the PIC. It should have been tea & bikkies and a slap on the wrist but........
If the principle of the 30 minute rule is to allow for 'visual maneouvreing below MSA in the case of an engine failure on departure', and it can be demonstrated that the commander did allow sufficient time for this then why would an authority prosecute for not observing an arbitary limit that took no account of the actual conditions?
if you would be happy to be stuck in the Queenstown basin with 20 or so minutes until ECT below MSA with any kind of weather in the vicinity, and an engine thats just gone bang, then either you haven't operated into NZQN in a jet or you are willing to bet everything (on behalf of your passengers) that you won't have an engine failure.
The chances are incredibly small that an engine failure will ocurr before the required height, but the company have assesed the risk and decided it's not one they want to take and written that into the ops manual.
Over the decades it's become obvious that if airlines leave every judgement call up to the PIC they will get all sorts of results, if they set some boundries (30 mins) they can get better outcomes safety wise. If our ego's are such that we think we're above that in non-emergency situations then we really shouldn't be in the left seat. My opinion only.