Good day Ian. I'll try to make this short and sweet....
Telling people that cannabis is "good" or "ok" or whatever because people have cannabinoid receptors in their brain is probably not a good argument. I was a paramedic a few years ago and I know that there are opiate receptors in our brains and spinal cords as well. The reason why heroine feels so damn good to users is because of those opiate receptors. Basically, cannabis isn't special in this regard.
So...to be consistent you would have to support using heroine. I completely agree with your positions on cannabis usage and drug "legalization" or "decriminalization" or whatever you want to call it. I just wouldn't use the receptor argument anymore because I know that heroine, morphine, oxycontin, ect. attaches to opiate receptors. In fact, I wouldn't be surprised if all drugs have some type of receptor in the brain.
To learn more just google "opiate receptor" and have a nice day. Thanks