The cannaboid receptors aren't really THC receptors. They are receptors for molecules that our body uses that are very structurally similar to THC. Because they aren't very specific (many receptors in our bodies are VERY specific for only 1 molecule), THC can also bind to these receptors and act on them. If I went back and actually read my neurophysiology notes, I'd have a much more intelligent answer. I see your point, but it's a happy accident that THC can utilize a receptor in the human brain.