Interesting, thinking about it more I wonder if part of it is also that airline companies are one of the easier things to tax too? Many African govs have a tough time taxing local citizens and businesses, but companies, often major ones that are probably multinational and have to strictly follow laws and have major assets that can be seized are likely a better target for high taxation (what are you going to do, not land at the one airport in the city?). Between that and a lack of competition, I bet that explains a good portion of it.