It's more about continuing the revenue stream from a dwindling product by transferring the tax liability to its replacement. Nicotine is the key factor, but its the delivery system that makes it attractive to former smokers. Nicotine gum and sprays just didn't do the trick to attract smokers, ecigs did. So now we have to take on the load of supporting tax laws that were designed to make an industry pay for its sins through monetary compensation to government.
Only, we are fairly certain that it wasn't nicotine that caused the ills and maladies of tobacco. Ironic, isn't it? It's kind of like taxing electric cars just because their fossil fuel predecessors ruined the environment and someone has to pay.
You don't even need to 'hedge' that with 'fairly certain'. It's tobacco combustion that does the damage - even most ANTZ will give you that.
And actually the car analogy isn't that close with electric cars and their fossil fuel predecessors, since the electricity at this point still comes from fossil fuels. One could argue it's worse since there are still coal plants whereas gasoline doesn't use coal (directly).