Maybe he was at your place before coming to mine!
You’re right we can all decide to provide extra amenities. But surely we can also decide not to in order to keep the price down. I’m sure my guest expected free bathrobe and slippers, included in the already low price, not to pay an extra fee for them.
It depends on what you state in your listing - do you SHOW slippers and bathrobe in the pictures? How much is your listing per night? Do you live nearby or do you have a property manager? Living nearby makes it easier to manage this kind of amenity, from washing them to making sure people don’t steal them…
I’m sorry but if I rent a reasonably priced whole apartment in a good location and the fixtures and amenities are as shown on the listing, I wouldn’t complain to the host about amenities THAT WERE NOT PROMISED in the listing.