Neither AMD or Nvidia can control who buys their cards other than feeding the OEM channel first, which is what AMD has done.IMO, the slight shift in GPU market share towards AMD only shows that they saw the opportunity to sell some GPUs by marketing them to miners, to the detriment of their loyal gaming fan base. Despite their press releases to the contrary, they did nothing to ensure stock was available to gaming customers, and all their recent cards have been very hard to find since launch. This was perhaps a smart financial move by AMD, but certainly at the cost of alienating many of their (previously) rabidly loyal customers. That's why I have to laugh every time someone says that AMD loves their fans and does everything for their customers, unlike those greedy evil NVIDIA scumbags. The only love is for money, on both sides.
I'm surprised miners haven't resorted to buying prebuilt HP, Lenovo or Dell machines with RX 580s and taking the GPU out and selling the memory and motherboard online. With the elevated memory prices it seems they could get the GPU cheaper after selling off the rest of the PC.