Its not the only defence, important and necessary, but also I think educating children about these risks in school is important (my kids' school does okay at this), and government regulation and criminalising such activity is useful too.
"Parenting" is a nebulous concept and at least to me, it also includes pushing for better laws for children. Nobody would think it's reasonable to allow stores to sell cigarettes to children and tell parents "if you don't like it, just don't let your kids smoke".
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.