The only entity that "owns" the software is Microsoft and it will never relinquish that, this is a fairly normal situation for software development, that still applies to this day for the very 1st version of DOS. HP or any other manufacturer do not own the software.
What they sell is a license to use the software and that is only to the end user. What the manufacturer buys is the right to install the software and sell it on to the end user, they never own the software or the license. It is covered by the End User License Agreement and the manufacturer is never the end user. A small manufacturer may buy the installation media itself along with the key and the right to provide it to you the end user, they cannot legally buy the license under those terms and use it themselves. Large manufacturers work slightly differently in that they notify Microsoft how many they have sold and pay the fees afterwards. Both do this on the basis that they provide support not Microsoft.
Windows 8.1 was a departure from the previous system in that it was an upgrade not an update for 8.0 users and free only to them. Any other upgrade requires a new key, 8.0 to 8.1 does not, the free upgrade was only available via the Store and was the 1st time this mechanism was used. This was not that smooth a process and we will have to wait and see how the free Windows 10 is released.