Previous unpenalised breaches of a rule don't entitle you to continued breaches.

It's probably because so many - including yourself - regularly ignored the cabin baggage weight limits, despite them being set out in the conditions of carriage on the ticket issued to you, that airlines like VA have had to enforce the limits. Qantas in Sydney domestic have scales at the entrance to the security lines and if any carry on is over 10kg, you can't continue on through security.

From a quick internet search, a carry on bag weight of over 12.5kg would put you in breach with almost every airline in the world which publishes cabin baggage weight limits except British Airways, Aeroflot and Frontier (https://blog.tortugabackpacks.com/ca...-weight-limit/).

As that blog post says, "The most common weight limits are 15 lb (6.8 kg), 18 lb (8 kg), and 22 lb (10kg), but you should always check with your airline before flying."