Thank you for the advice @Helen. Actually I have already had professional photos taken and I am using them, of course, so I don't have a problem with generally getting a photographer and the quality of my photos. The main reason I would like the Airbnb professional photography feature is the Verified seal I mentioned already. If you click through your professional Airbnb photos, you may see it on the bottom left of each Airbnb professional photo: the words "Verified photo". As I mentioned before, it is very important to have this, among many other verifications, in order to encourage guests to book your home as they know that a representative of Airbnb has visited the home and verified the images representing the listing. This is a huge plus in a market that is riddled with fake listings and scams.
Further, speaking for other hosts who may be "hung up" on Airbnb professional photography, maybe they don't have the resources to pay for a professional photographer (especially if they haven't rented their home for extra income yet!).
And besides all that, even though we appreciate the free photography that is offered to some listings, if a company is going to advertise a service as what sets them apart from other companies, the least they could do when they deny you the service would be to give you an honest answer as to why. I am sure we could understand if it is not economically viable for them to shoot all listings!