I feel for you @Mark769. We just had the same thing happen to us, and it's so frustrating. I explicitly had the conversation with the guest in advance of accepting their one-month booking to make sure they understood the requirements to enter Canada and our cancellation policy. This was amid the Omnicron surge, so I thought it was risky of them to book overseas and wanted to do what I felt was my due diligence. Sure enough, three days before their arrival, one of the guests tests positive for COVID, and they can't travel. Airbnb emailed me at 2 am to notify me, then at 5 am, they said that since they hadn't heard from me yet, they would cancel the reservation. I was shocked! When I responded, they said they couldn't help it and couldn't support my time zone or the time I woke up.
I certainly don't want people travelling if they are ill and agree with @Pat271. However, the rules to enter a country are known at this point. The risk of contracting COVID is high and broadly stated. If a guest is willing to accept those risks by travelling, I think they should bear some of the risks. My month was reserved for them; it's less likely we will replace that booking in the short term.
@Nanxing0 I thought I recalled in a recent Airbnb update mention of Airbnb travel insurance, which is how these situations should be handled at this point in the pandemic. I have compensated many guests over the years for personal extenuating circumstances, but the COVID play is getting a bit tiresome at this point.
I called to make a complaint and was invited to provide feedback at www.airbnb.com/help/feedback. I'm sure it goes into an abyss. However, if you feel sufficiently frustrated or inclined, opinions about the policy can be shared there.