There are built in feedback mechanisms to encourage responsible behavior from both parties which makes it less likely that someone would use AirBnB to commit "illegal" acts (not entirely illegal in all markets). You're being down voted because here's why it doesn't make sense to use it for prostitution:

- Many AirBnB listings are shared spaces so you're getting a spare bedroom or a couch/futon so the privacy factor is not there.

- You have a traceable identity on AirBnB so if it was illegal, you would be leaving a trail of evidence pointing to you.

- Your host would most likely ask you to leave and then you would get banned from the site or get a really negative review which would prevent you from using the service again.

Mostly, it's just not something that is going to be common enough to be a big problem and like I mentioned, there are built in feedback mechanisms to discourage bad behavior.