"And I do not see how making AirBnB's computers "kill" a profile once they have passed 120 rented days is not making AirBnB enforce the City of Boston's laws. Whether it is a computer, a person, or a monkey with a marker, the act, not the mechanism, is what matters in determining enforcement.
Also the bar analogy is completely specious. A bar patron buys a drink from the bar. AirBnB connects two parties, one who has a property for rent, the other who wants to rent a property.
My original point still stands. AirBnB isn't fighting the regulation of STR, it's fighting government imposition of its day to day operations."
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Once the person has passed 120 days of rental, later rentals would be illegal. AirBnB knows that, because AirBnB tracks usage (tomake sure it gets paid). So AirBnB is knowingly facilitating an offense. It is Boston's right to prevent that. That has nothing to do with "day to day operations." You can't facilitate transactions you know to be illegal.
As for the bar, let me remind you that AirBnB takes a cut from the transaction. It's not just connecting two parties. It brokers them, and profits from having the listings, if only to play up network effects to get other hosts to post on AirBnB. Is it a great analogy? No. It does point out, however, the right of government to place burdens on a business to protect the public weal. Certainly selling drinks is a bar's "day to day operations."
As for getting the information, AirBnB is doing business in Massachusetts, and Boston in particular, so the question becomes whether Boston's demands are reasonable. Making sure that AirBnB only does business with registered property hosts -- by having AirBnB turn over host lists -- is reasonable. If AirBnB does not do so, then AirBnB is potentially facilitating illegal transactions, and will be fined for so doing.