The problem they will have with taxing cannabis compared to alcohol and tobacco is that there is already a thriving underground distribution system set up right now.
There was one for alcohol during prohibition too. Most people now buy their alcohol in stores though.
But if weed prices get ridiculous because of the taxes people will just go back to buying from dealers.
Which should become easier for dealers and growers if we have legalization. "I smell marijuana" will no longer be probable cause to perform police searches so even dealing and black market growing will be easier
If the government was smart about it they would set up loooow tax rates on cannabis and offer fair prices, then as time goes on and it becomes common to buy from a dispensary and it would seem weird to buy from some guy growing in his basement they could start raising up the taxes to make their money off of it
Our politicians are elected by the American people. In a democracy you get the government you deserve. That's why our politicians are stupid. And they probably will overtax it at least at first. Eventually growers will form real lobbyist groups and have enough power to lower those tax rates, but that will take time.
Legalization is NOT all good. It's not perfect. It'll fuck over some people and make others rich. But it'll also bring us more freedom and public acceptance. It'll stop people from going to prison for growing a plant. IMO that should trump all other factors.