It is definitely possible for the host to pre-screen and validate the guest's identity. I've used AirBNB as a guest before and I was screened by hosts. They ask

It is definitely possible for the host to pre-screen and validate the guest's identity. I've used AirBNB as a guest before and I was screened by hosts. They asked questions about why I was in the area, whether I smoked/drank, etc. I told them my name, allowing them to look me up on FB, Twitter, find my personal blog, etc. You'd better believe I looked up their names as well to make sure I wasn't inadvertently staying with someone who was a felon. If your argument is based on the assumption that hosts can not screen guests then your argument is wrong.