Maybe they do. Perhaps customs can't always tell. I've certainly known them to knowingly allow in small quantities and things that are obviously for personal use. Besides I didn't say I agreed with the law, but it does exist.Customs ignore this rule all the time. I see people get second hand clothing in parcels whenever I go to Cape Mail and it's not their own.
Maybe they do, maybe they don't. Maybe that's an old regulation that no-one is bothered to get rid of. It's another one customs do quite often choose to ignore. But at least they don't make it difficult to register.In SA if you do PERSONAL (not for profit or resale) internet shopping for more than 3 separate items per year you must be registered as an importer. No other democracy in the world has such a ridiculous rule.
South Africa is hardly unique in having many silly, pointless laws that the authorities quietly ignore. They should be repealed, but lawmakers have never been big on doing so, which is why it is always best to fight bad laws before they get enacted.