Problem is Van is that RSA have spent a fortune on Air Defence - they just don't have the right sort to stop this sort of thing.
The other is that infrastructure is spread over a very wide area - it's not like defending an airbase or a city - a $1000 drone can take out an electricity pylon in the sticks and close an installation down - and they can do it the next day 20 miles away etc etc etc
The "entry costs" (as an Economist would say) are so low and the technology is so simple it means just about anyone can get involved. See the idiots trying to close down Heathrow this weekend - 5 or 6 protesters who can afford to buy a couple of drones and operate them from a field........