There are a few points in the article that are not explained fully.
Firstly (IANAL...) windows can typically be purchased in two flavours - an OEM (Original Equipment Manufacturer) license or a Retail License. There are others that typically affect businesses - like Volume licenses where the company negotiates to purchase a large quantity of copies of Windows (or Office etc etc)
Computer builders usually include an OEM license for the main reason of price. OEM licenses are quite a bit cheaper than retail licenses - which allows the builder to either sell the computer cheaper - or make more profit.
Users who wander into a retail outlet will usually see on the shelf, fancy boxed copies of windows - these are the retail copies.
The difference between the two - OEM licenses once installed are then associated with that one PC forever. You are not allowed to uninstall an OEM license from one computer and install it on another (newer?) PC. The OEM license is not transferable to another computer.
Retail Licenses you may remove from one PC and install on another PC (but only one at a time).
The second point that Microsoft started to bundle "diskless" copies of Windows with new PCs that are shipping so Microsoft could make more money - I doubt. If they wanted to make more money they could just drop the OEM license altogether and require every license to be the more expensive Retail license.
The reason the OEM license exists, is because many PC Builders include other programs (many that are unwanted by the user), custom drivers, wallpapers and other files to customise the OOBE (Out Of Box Experience). Allowing PC Builders to configure a custom version of Windows to show the new PC in the best light possible to its new owner/purchaser. They also typically include utilities to create recovery/setup disks so you can return the PC back to it's original setup (if things go really wrong).
So shipping PCs with a custom copy of windows on the hard disk is less about saving Microsoft money, and more about making life easier (and more flexible) for the PC Builders. Conversely - life as a result isn't always "easier" and "more flexible" for the owner of the PC.
As for what is legal/illegal when selling a PC with software. If you include the Retail license on the PC you are selling - you cannot keep it for yourself. You cannot use the OEM license of Windows if you sell the PC (even if you remove OEM windows from the PC you are selling).
Some software licenses include a legal clause along the lines that the software is licensed to you and you are not allowed to sell/rent/giveaway the software to anyone else (even if you were to give up the license/possession of the software). I'm doubtful such a clause would stand up if tested in court (for some countries). Some license agreements can be pretty outrageous - but nobody reads them, other than as a cure for insomnia.