I think its funny that something that is an obvious saftey mod can make a car unroadworthy. My uncle had a similar problem when he was putting VT disc brakes in his LJ Torana.
I can see your point - but the thing is that there's lots more to consider other than just 'it's got a cage/bigger brakes so it's safer'
I think the biggest cause of injury in a car accident is caused by contact of the occupant(s) with bits of the interior. - with things like a rollcage in a street car - you have seatbelts which still let you flop around like a dummy when you crash, and the car is designed with that in mind. - There will be clearances, padding, extra chassis/cabin rigidity in places... and all that kinda stuff that they discover is needed after they do a whole bunch of crash tests with multi million dollar measuring equipment etc.
Now the problem is - if joe blow backyard rollcage welder knocks up a $200 cage and just bolts/welds it into that car - it can very easily make the car a lot more dangerous to the occupants than if it had no cage in the first place.
This is why proper cages cost into the thousands, and require a andra/cams engineer inspection and approval to be used on the race track.
I can see their point with brakes and stuff as well - the torana would have brakes and bracketry made to suit it's weight and all the loads that the guys at the design shop would have worked out that it will be exerting on these components. They also have to put up with a whole bunch of heat and make sure they clear all the other suspension components properly etc etc.
Unfortunately the roads authorities don't trust all of us to do the job properly - so they require an engineers certificate to prove that it's not more of a safety risk than an upgrade...... and I completely understand that having seen some pretty dodgy 'safety mods'