This is arse about face. The brain has many places where bodily produced molecules perform some function. The receptor is shaped to accept only that molecule, like a key in a lock. It's how ordinary painkillers work.
It just so happens that weed molecules fit one of the receptors.
The researchers named the receptor after the thing they were examining fitted - it could just as easily be a molecule found in kiwi fruit.
I believe they aren't sure what function the receptor fulfils in standard body processes.