Because they aren't really implementing the WinAPI, they're reverse engineering it and making a similar one that tries to mimic it without ever using more than the details Microsoft makes available and their own debuggers.
There aren't patent problems because you can't patent the concept of that API or it's functions (ie. You can't patent a button because it's not specific enough, and if they patented the way it worked then they could program it a different way).
And there aren't copyright problems because they don't use any of Microsoft's code (and if you've worked with MS's code you probably wouldn't even be allow submit any code to WINE).
But back on topic, I'm not going to install Mono or Moonlight on my computer, and since Mono seems to be working it's way into a good few core GNOME apps, that means I won't install GNOME, that's one of my reasons for using KDE (The others being that I prefer QT and it seems better for developers). If there was an option to not support OOXML in KOffice then I'd not use OOXML, but I suppose I'll have to wait and see about that.
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