If you own a property (or have the owner's permission), why does it matter whether you actually live in it or rent it out to other people? Should we be prohibited from lending our physical books to other people just because publishers would rather we didn't?
Why does it matter who you rent out your property to? Why does it matter whether you rent it out for 30 days a year or 300 days a year? Why does it matter whether you rent it out for 30 days to one person or one day each to 30 people? The last one is especially arbitrary, and the only reason it means anything at all is because the hotel industry uses it as a threshold for market segmentation.
Is it safety that matters? If so, let's drop all the "up to X days" bullshit and actually focus on safety. How can we check whether an AirBnB listing is "safe", whatever that means in this context, how do we compensate victims of "unsafe" listings, and how do we come up with a good incentive structure to encourage "safe" listings and weed out "unsafe" ones?
Is it taxation that matters? Of course people should pay taxes on all of their income. In fact, AirBnB income should be easier to track down than the proceeds from that MacBook you just sold on Craigslist. AirBnB keeps excellent records of all of your income (and their fees), after all. Then it's just a matter of getting those records into the hands of the IRS.
Is it zoning that matters? If so, why do we care about zoning at all? Over the last few decades, the only thing that zoning achieved in the United States is the complete segragation of residential areas from all other areas of modern life, so that everyone needs to drive a dozen miles to work and another dozen miles to get their groceries. Many cities are now beginning to toy with the idea of vibrant mixed zones, where people can live, work, shop, and play all within easy walking or cycling distances. A hostel next door suddenly doesn't seem like a bad idea after all. (Oh, it's not properly managed and people are shooting crack up their arms in the backyard? Well, that's a problem with poor management, not a problem with the mere fact that it's a hostel. Would you ban houses with basements because some basements have meth labs in them?)
So I refuse to believe that the right answer has anything to do with how long you should be able to rent out a property. It's your property, you should be able to do whatever you want with it within reasonable bounds (like paying taxes and keeping your property safe for other users).
As long as we keep focusing on peripheral issues that are framed by existing business models with conflicts of interest, we'll never get to ask the really pressing questions, such as reasonable safety and insurance standards for small-time landlords.
And yes, I agree with you that AirBnB's reactive (as opposed to proactive) attitude toward this issue isn't helping, either.
/rant