@Mike-And-Helen0 I absolutely think they shouldn't host without their landlord's permission- that's illegal, apart from being rude and inconsiderate and disrespectful. As a homeowner, I would be livid if a had a long-term tenant who was listing the place, or even rooms in it without asking my permission.
If a tenant is listing without the landlord's permission, if the guests stop up the plumbing, do you really think that tenant is going to pay a couple hundred bucks or more to hire a plumber? No, they're going to call the landlord, and expect them to pay for it, and never mention that their heretofore unknown Airbnb guests were the perpetrators.
Another thing that's quite common among these types of hosts is that they instruct the guests to tell anyone who asks that they're just friends. It puts the guests in an uncomfortable situation to be expected to lie on someone else's behalf.