If it's efficient breach than surely AirBnB sublettors violating their inefficient leases are compensating landlords for the breach making both parties better off. Right? Surely it's not one party secretly violating the terms of the contract and keeping all the benefits? That doesn't sound very efficient at all; don't think that's what Posner had in mind.
Landlord has been using the same faded form contract for a decade; doesn't even know for sure which provisions are in it, or still legal under recent legislation or court decisions. (But, a standard 'severability' clause protects them when they've got illegal and unenforceable terms inside their template.)
Renter skims contract, but doesn't consider its terms negotiable, practically speaking. They often ignore the precise terms regarding notices, houseguests for more than X days, small pets, drugs on the premises, new roommates, sublettors, etc. In many cases the local law, or tenants' legal support groups, will defend them for violations of such terms - because the signed lease was illegal or unreasonable.
During the course of the lease, both the landlord and the renter may deviate from the exact terms from time to time, by tacit agreement, or occasionally verbal agreement (even if the contract stipulates "no verbal amendments").
Many renter violations are essentially in the realm of "don't ask, don't tell". The landlord doesn't really care and also doesn't want to know, because knowing would force conscious acknowledgement, perhaps encouraging further violations or incurring liability. Only if there's actual property damage or risk of harm/liability (or other adversarialism has begun from other factors) does the landlord become interested in monitoring and enforcing every clause.
Into this mix comes AirBnB. Many landlords won't notice the activity. Or they'll suspect it but don't want to pry. Or they'll know, but don't want to be known to "know".
Yet they may notice the rental rates the market will bear go up, benefiting them for looking-the-other-way. They may notice the renter's treatment of the property improving – as they now share an extra interest in its appeal. Compared to an empty room/apartment, the presence of good guests can make the property and neighborhood a safer and more pleasant place. So landlords can benefit implicitly even without an explicit renegotiation of terms.
In fact, they may prefer this sort of indirect benefit – because they retain the optionality of enforcing the terms, if things go poorly, without the risk of formally approving the activity.
Real economic and social relationships are way more complicated than, "the letter of the rules is X so everyone is duty-bound to do no more nor less than X".
