You should consider a thing: a host offers his space because he likes people and because he needs the money. So for most hosts, a guest not coming for a long stay and deciding that on the last moment would create a problem. Hence the strict coancellation policy - and also the possibility that he offers to give a partial refund if he can find another guest for part of the cancelled stay.
Other hosts offer flexible or moderate conditions, for the simple reason that they will always rent the space again if it is cancelled. If someone is in a good location where he can always rent, even on the same day, then I'd suppose he will ask for higher prices.
So I would think it fair, if you know already that your plans may change to discuss that in advance and to take a travel insurance.
Your plane or train ticket will also cost less if it is fix and be more expensive if you take a ticket which can be changed or returned.