The reason Microsoft allows us to download these .ISO files for free is because, if you buy your pc, you most likely have Windows 7 on it, so for whatever reason that you want to reinstall Windows, you just have to enter in the activation code on the back of a sticker on the back of your pc. And because the copy of Windows 7 on your pc is legal, it is ok to enter that code into that new copy of Windows. Hope this helped! - LikeAxis33
Actually, you're not supposed to use those links - they're for people who bought Windows through the Microsoft Store as a download. I don't see a problem with it though, as long as you already own a legal copy of that OS.