I know that you can run Windows on a mac, but you can't use OSX to develop for Windows.

You can't even install OSX on something without an Apple logo (well, in theory you can but you shouldn't because insert legalese and threats here).

In the case of Apple their closeness is not hurting them, and definitely not decreasing their adoption.

In the case of Microsoft... well, they still need to convince people to take Windows Phone seriously, so their closeness only helps to hurt them.