Fixing a bad law is fine, but you can't propagate that back just because you wish you'd thought of the edge cases then. Or at least you shouldn't, if you have any sense of fair play...
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".
Either the Host does, or the Consumer of the service does, AirBnb would just be collecting and remiting that on behalf of the parties
It’s an interesting thought exercise, and fairness would in my sole opinion be based on whether they knew they should have been paying the whole time and were getting away with it. It won’t happen, though.
Tax law is what's written in statutes, but it's also how the courts interpret the law. This means there's stuff that's clearly lawful, stuff that's clearly unlawful, and then a grey area where reasonable people can interpret the law in different ways and we need the courts to rule.
This works well when you have cases that go to court. But some tax authorities prefer a light-touch or right-touch regulatory approach, and so they prefer to persuade people to pay tax that's owed and they leave court cases for the blatant offenders. That means we don't get the case law.
What's being explicitly applied is the federal and provincial sales tax.
Airbnb and the hosts were selling a service without collecting the sales tax.
Someone was tax evading, and someone owes the CRA back taxes.