You seem to be of the belief that a) financial institutions never play the limited role of facilitator and b) entities which facilitate transactions are somehow less culpable when they know that the transactions they are facilitating are ethically questionable. You're wrong on both counts.

Many financial institutions facilitate transactions. Goldman Sachs paid over half a billion dollars to settle charges related to the Abacus deal it facilitated in 2007.[1] Goldman was paid a $15 million fee for facilitating that deal, a meaningless fraction of the $46 billion in revenues it earned in 2007.

As I noted, the vast majority of apartment leases forbid subletting without the landlord's approval, and you would be hard pressed to find a single landlord who allows tenants to turn their apartments into hotel suites. As such, one could argue that as far as apartments go, this isn't a "might" proposition. There's a very reasonable argument to be made that AirBnB can safely assume that apartment listings on its site more often than not involve a tenant breaking the terms of his or her lease.

If you don't have a problem with AirBnB facilitating apartment rentals in the face of constructive knowledge that many of them are taking place in violation of the terms of the host's lease, that's fine. But then you shouldn't have a problem with financial institutions when they merely facilitate transactions that don't pass your smell test. What's good for the goose is good for the gander, right?

[1] http://sevenpillarsinstitute.org/case-studies/goldman-sachs-...