You only hear about this after your booking has been accepted -- in the case of instant booking, you have already been charged. And it's usually not too much -- maybe 20-40 EUR. Often it's explained as a mandatory city tax or something like that, and you don't know the city so....
So at this point you can choose: report the host to AirBnB, lose the booking you wanted after already going through all the stress of finding it, and wait probably a long time before you get your money back.
Or, just grumble and pay it. Because of the way reviews are incentivized, it would be a Bad Idea to complain about it in the review, which is why you didn't see anyone complain about it so far.
I'm not proud of this, but the four or five times it happened I just paid it, because in the big picture it wasn't that much. I think the highest was 100 but it was for a long stay costing over 1000.
But unlike the "cleaning fee" where you can, with a lot of imagination, picture some definition of "cleaning" that might, on average, cost that much -- maybe every third guest soils the bed? -- the cash-on-arrival thing is transparently cheating.
Follow the incentives -- five star reviews only, please!