There are no particular bylaws governing subletting in tenancy agreements, but almost all landlords forbid it. Mine does. Yours probably does, too. AirBnB has not, and will never do any due diligence in verifying that a renter's tenancy agreement permits short-term subletting - they prefer remaining ignorant of whether or not their units are legal.
[1] http://www.nolo.com/legal-encyclopedia/overview-airbnb-law-s...
There also exist services to ensure compliance - http://www.hostcompliance.com/
A quick google news search also turns up a lot of articles about cities banning / deciding on the legality of abnb.
Additionally, your post was likely downvoted because you dashed off an aggressive, cliched comment that could have been answered with less effort than it took to post to HN.
Renting out residentially zoned property on AirBNB or shooting porn?
But the company has historically never had qualms about screwing their customers over, so I wouldn't hold my breath.
Small legal point. Contracts are not allowed to impose fines. They can cite liquidated damages (what most people see as a fine) but they have to be rationally related to the damage done by the activity described. They cannot be arbitrary or elevated simply to deter unwanted behavior. Yes, this is and issue when it comes to "parking tickets" on private property.
500$ fee to accommodate a cleaning crew = 0k.
5,000$ fine for pain and suffering caused by naked people in someone's home = not enforceable.
After all, isn't that essentially how overdraft fees work? "Oh, you want to withdraw $2 when your balance is $1? That constitutes a request for a loan, for which we charge a $35 fee."
It might have a liquidated damages clause if the use of and stipulated amount of liquidated damages was such as to be found reasonable for the type of breach it covers; otherwise, a "fine" in a contract is generally an unenforceable penalty clause.
In the case of non-damages, what recourse do you really need?
The "loophole" is there is no way to sue when the host is illegally subletting his apartment at first place. And a good bunch of Airbnb rentals are straight up illegal. Airbnb is a legal nightmare both for guests and hosts. Most of the time everything goes fine. But when things go wrong Airbnb is effectively of little help. That's the sharing economy, the risk is passed down to the user of these services, just like Uber, Kickstarter and co.
I am very wary of companies that try to 'help' by requiring 3rd party private arbitration instead of taking it through the established legal system.