Which of course, they are doing on purpose. They can't ignore the install base of Windows Desktop PCs, but they don't want to give up any sales to WP.
I had posted earlier in another thread that Apple has to be figuring out how to compete for the future because their ecosystem of "everyone needing the iPhone, iPad, MacBook, etc." wasn't going to pan out long term. They made sure their products didn't step on each other's toes so they could sell them all to the same people, who would need them all to get things done. Now, with Windows 10 and W10M continuum, people that want to buy only one device will be able to get away with it and Apple will be left with unsold hardware if they don't make some changes.
I believe spreading their services to other platforms is exactly the kind of move they are needing to make in order to survive the next wave of technology changes. It's a good move by Apple and will help keep them relevant as things move forward. Smart move. I expect more services to be ported. People with iPhones who have Windows Computers will be happy for this, and would also be happy I'm sure to receive other Apple services for their Windows PC.