- 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.
Assumption made here is people will report and provide review comments. if they (who want to do illigal stuff) select places to get one room and behave normally and decently in public, nobody will ever report anything. I am not saying it will be a widespread practice, but all it takes is one dead fish to dirt the pond.
And nobody answered the first part of my question. ie the factors that promted or inspired PG to make that remark when he visited them.