Why should it be illegal?
People should be able to do whatever they wish with their bodies. From a health perspective, THC usage has been linked with fried short term memory. It should be used carefully like most medications and warning labels have to be printed, just like for alcohol and cigarettes. If not on the package, the seller should have to inform a new client of the risks.
Once the client has been informed they can buy as much as they want. Smoke as much as they want. Shove as much as they want up their anus. Why should the state be involved?
Yes, exactly if someone wants to take something that will outright kill them in 2 seconds, they have the right to do that. Your body is your body and no one else has the right to tell you what to do with it as long as you aren't hurting someone else by ingesting it.