If you install windows 10 as a "clean install" without upgrading first, your win10 version wont be activated unless you buy a new win 10 license. If you already own win 7 or 8.1 you must UPGRADE from that activated version to properly activate YOUR windows 10 license.
What Microsoft has done with windows 10 to allow the "upgrade" thing, is that when you upgrade a proper activated win 7 or 8/8.1 license to windows 10, their (microsoft) servers will save your hardwareID (from your macbook or whatever computer you use) as a ID saying your windows 10 license is genuine.
If you dont do this FIRST, then you have to purchase a new license.
So - Proper way to get a free license of win 10 is : Install win7/8/8.1 -> THEN upgrade to win 10. After win 10 is saying "genuine" and are activated, you can delete the whole bootcamp partition from OSX and create a new win 10 bootcamp installation from scratch. It will then check your HardwareID (as described above), then determine that "Hey, your hardwareID is genuine" and then its activated 🙂
C