Why is it the worst example? Sure, the laws don't 100% prevent children from smoking, but certainly they have an effect. I'm not using it as an example of a perfectly effective law, I'm using it to make the point that it wouldn't be reasonable to say "parenting is the only defense against smoking" and scrap the law.
It's a bad example because the last 5+ years in UK/US have seen an epidemic of child vaping and nicotine addiction when it was illegal for them to have it, while the Tobacco companies fight tooth and nail against regulation on fruity flavours and bright colours.
I don't want to scrap laws. In Britain we're 20 years ahead of the US on this (see Gambling Act 2005). Gambling laws are written by the gambling industry so no parent should think that the law will do anything to help their child.