Jabs only weigh 350-370kg empty which is why there’s no angst caused.
The issue with them registering an RV9 as a 2 seater is that once it’s in the system it’s hard to get it out, especially once it changes hands. If you don’t fly it overnight, the next guy probably will, as has happened before. They have to draw the line somewhere and or work to a formulae. Problem being that if they implement a formulae, it won’t please everyone and next they’ll be trying to withdraw rego on a converted 150 or a Colt or something and the cycle begins again.