I guess the point is you might already own a PC, and you could use that for developing Android apps, for example. But if you also want to use Xcode and develop iOS apps as well then you have to buy a Mac.
No, you don't need a Windows license to develop for Windows. That's the whole point here. You can code and compile binaries for Windows, Linux, Android, etc. from many IDEs and virtually any platform under the sun. You can code and compile binaries for Apple products from exactly one IDE and exactly one platform: Apple's.
You might already own a Mac.