[chrismorgan]

> because Microsoft gives out windows isos for free,

… with a license that only allows you to use it for certain purposes, subject to certain restrictions.

> and the seller is really selling a random 25 character string, which can hardly be copyrighted.

1. Copyright is about creative works. It is possible to have a meaningful creative work no more than 25 characters long (or equivalent). Music is particularly good at this.

2. The key itself is not copyrighted (it’s not a creative work), but is reasonably interpreted as a copyright circumvention device. See also https://en.wikipedia.org/wiki/Illegal_number.