because protectionist policies of governments prevent that plane from doing a local return trip when it would otherwise be idle. Except in the EU airlines do not have the "seventh freedom" and we pay higher fares because of it.

In the example you give the BA aircraft could fit in a Jo-berg-Nairobi return trip rather than sit on the tarmac. It is prevented from doing so by the The ZA and KY governments who would prefer to protect South Affrican and Kenyan airlines from competition.