Personally speaking, i don't think much blame can be put upon the referee, it's a poorly written rule that is open to so much on the spot analysis, hard to spot in the split second it took place and has any photo actually proven that ZC crossed the line, bearing in mind the rulebook does not clarify which is the edge of the track and how much of the wheel has to cross it to mean an offence.
I'd say the rule should be rewritten along the lines of the wider part of the whiteline is the marking for the inside of the track, if both tyres touch the inner edge of the white line or further onto the offtrack area then an exclusion will be given unless it was due to taking avoiding action etc.