I think the difference between Airbnbs and couchsurfers is huge. Couch surfing is a voluntary yet free service provided by someone out of the goodwill of their heart. Airbnb is provided for profit. Leaving such reviews is fine for an Airbnb (assuming it's deserved ofc), but certainly not okay for couch surfing.

It does depend what they've reviewed though. Is the person hosting living in a in a gross apartment vs the towel designs are not nice.