Maybe they thought airbnb is like couchsurfing 🙂 I was checking out couchsurfing in my area (just out of curiosity) and found one listing where the guest was looking for someone who was willing to host him (hosts receive no payment whatsoever for hosting a couchsurfing guest) for an entire semester. The guest wanted the couchsurfing host to provide a private room and desk so that he could study and have privacy. (WTF?)
In Korea, there's this widespread problem where bloggers with a ton of followers try to eat for free by telling the manager/owner that instead of paying they will post a great review of the place with pictures and if the manager/owner doesn't accept this, they hint that their review might not be as good as the manager/owner hopes.
I think you handled the situations like a pro. I really wish Airbnb was more active in promoting responsiblity of guests as much as they do to hosts - like you said, I also report guests who I feel are inappropriate, but I don't think Airbnb does anything about it. I also completely agree that government IDs should be mandatory. A lot of us are opening up our homes to host guests, and guests are paying to stay at the home of a stranger. I understand no amount of screening will be perfect, but the least Airbnb can do is to at least pretend to care enough to TRY and do a better job.