TL;DR – Legalization of cannabis is just as much about harm reduction as it is about freedom.
The FDA takes years to ban a substance. We are really good at chemistry these days. As soon as a substance is banned, we can create another one that is almost the same as the banned one.
Unfortunately, this results in a situation where it is impossible to keep up with the bans on synthetic drugs (such as spice), and easy to ban natural drugs, such as normal cannabis.
It reminds me of how countries that crack down harder on prostitution also have higher STD rates. For example, in Kenya, you can be arrested for carrying a condom because they assume you are a prostitute.