The problem is that it's hard to tell when value arises from the elimination of a deadweight loss and when it arises from externalization of costs. E.g. environmental regulation generates some deadweight loss in the form of compliance, etc. It also seeks to internalize inherent costs of activity that would otherwise be externalized. If we got rid of the regulation, a factory owner would see a small amount of benefit from the elimination of the deadweight loss, and a very large amount of benefit from being able to externalize the inherent environmental costs of operating a factory on the local community.

In the situation of AirBnB, the externalities are imposed on all the other residents of the building and the neighborhood who signed up for apartment leases on the assumption that random people would not be coming and going. The value that accrues to the AirBnB host is generated partially by the loss of value to his neighbors. So what you have is not a creation of value, but rather the opportunistic transfer of value.