So the fact that we already have one awful policy (legal tobacco) is not sufficient to justify implementing another one. Marijuana seems to have roughly the same or slightly lower impact on lung cancer as tobacco (hard to measure since most people smoke both). Of course it has other harder to measure effects like long-term brain damage, and DUI risk, or even loss of economic productivity and workplace accidents.

The US (and most of the world) has been triumphantly marching towards banning smoking and yet we seem to be normalising the use of another substance that isn't any better. It seems likely that we will be in the same place with marijuana in a few decades as we are with tobacco.

Edit: I realise that you may have not read my connected comment. Taxing tobacco doesn't make the government money, lung cancer from tobacco smoking directly costs Medicare 4x the total tax revenue from all tobacco products. So that is my basis for "taxing legal tobacco is a poor policy" and by extension marijuana will be as well.