To me the issue is scale. Just like people can hold garage sales or lemonade stand w/o a license vs running a bar.
By same token - I don't mind someone renting out AN apartment. If the issue is tax - Airbnb can simply collect it on behalf of the renter. As far as consumer protection - the guest deals with AirBnB and not an individual host.
If license was obtained by simply filling out a web form and paying a fee - I'd be all for it. But the way things are now, the whole issue of licensing is about protecting the hotel industry.