When they really need to. But they'll fight it hard! Apple's business model is to lock everything down -- very closed indeed. This does not align with the goals of bitcoin, at least, how I perceive them.
tl;dr Apple = closed. BTC = open.
If Apple was to even accept cryptocurrencies that will not be Bitcoin for sure. They will maybe create their own blockchain and fill it with their own native token. I believe that this is imminent to happen in the near future for many companies like Apple, Amazon etc. They will have more earnings from a move like this one rather than integrating an already developed cryptocurrency that they are not in the control of it.