Why should they increase their prices, not saying amazon is an ethical company at all, but their prices are already cheaper than in SA, if they keep their current pricing they will still make whatever profits they are making anyway.
Just to add to the reply by
@ToxicBunny:
In most geographies retailers source their product from local distributors, not from some central global warehouse. For various obvious reasons (shipping, volume discounts, duties, local taxes, employment laws and costs, etc) those distributors have different prices.
So, Amazon in SA will typically source the bulk of its items from the already-established SA distis.
Mainly because order and shipping volumes in SA are much smaller than in larger markets such as the US, landed costs before duties and taxes in SA are higher than in the US. That waterfalls down through the channel to the retail customer.
Also, vendors and regional distributors often run regional and market-specific specials with deep discounts in order to rebalance inventory. Those promotional special prices are not available worldwide but are limited to certain geographies.
I spend about six months a year in the US, and Amazon here is absolutely flippen amazing. They never cease to astonish me with speed, price, service and value.
But I wouldn't hold my breath that prices in SA will match those in the US, or that deliveries will be any quicker. For a start, our market is too small and too far away from manufacturers to get comparable pricing for bulk purchases and shipping.