The problem is liberals will ignore that smoking it causes lung cancer.
The problem that conservatives will ignore is that cigarettes are much more likely to cause cancer because people consume far more of them, and they contain many more carcinogens that pot. Yet, I don't hear any calls to ban cigarettes because doing so might anger their friends in big tobacco, not to mention about 20% of their constituency - more in some red states where people are more likely to have poor health practices.
Another problem that conservatives ignore is the fact that banning a relatively harmless drug - and the war on drugs - is just more big government trying to protect us from ourselves with the nanny state mentality. I'm willing to bet that many of the people who want to maintain a ban on pot are the same ones who have a problem with Michelle Obama talking about nutrition and exercise. Do you see the hypocrisy? Either you want government concerning itself with our health or you don't.