Again, take my Windows 10 example.
You can download it without buying it and it will not be considered as piracy. So why this would not apply for Nintendo Switch games?
This is the same freaking thing!
FFS!
If you download a legit win10 copy somewhere from the internet, it WILL ask you to insert a product key, if you skip this, windows will work but IT IS NOT a legit copy of Windows.
If you created an install device like usb stick or a dvd with the windows tool from their own website, you will paste your own win10 key within that installation so you won't get asked to verify your copy again, because you did when creating the copy.
Windows is aware of the 'legit install' of their os, if they want, they could probably put you down and sue you. But it's a cost/profit ratio not worth for a single user!