Laws are written because if there is no law some clown would take it to the nth degree.
If 30 Minutes (not much IMHO) was not written in law, some idiot would consider that 5 minutes was enough. There are two many clowns in the world to leave it up to self regulation. Some people just need protection from themselves.
I agree with mikewill, after a very long time in the industry, I would consider it highly unlikely that CASA would ping you for having 29 and a half minutes left in the tanks. FFS how would they know, it only a litre or 2, pretty sure the dipsticks for light aircraft are not that accurate.
If you landed with only a couple of litres, thats a different story, you would definitely need to explain that one, and so you should have to!