[usrusr]

Legislative changing laws, that should definitely not apply retroactively, even if sometimes this is very painful.

Executive fixing application of laws that haven't been applied correctly in the past however is an entirely different story. If laws oblige you to pay taxes you are obliged to pay wether they have been collected or not. Not much different from a crime being a crime wether you are caught or not. Unless of course there's also a law that states an equivalent of "taxes not collected within the first $n $timeunit are waived".

[syshum]

The problem with that is the AirBnB never owed the tax in the first place.

Either the Host does, or the Consumer of the service does, AirBnb would just be collecting and remiting that on behalf of the parties

[Sanzig]

Depends on the jurisdiction, but most wouldn't put this on the consumer. In most jurisdictions, if a merchant neglects to collect sales tax when the sale is made, it's out of their pocket - not the customer.