By the way, this is a relatively recent change made in July 2017 (License version 10). Before that there was no "noncommercial" restriction, and the free license allowed commercial use as long as you installed it yourself (i.e. not deployed company-wide etc.).
Old FAQ, license, and a relevant FAQ excerpt:
https://web.archive.org/web/20161108235524/https://www.virtu...
https://web.archive.org/web/20170126144347/https://www.virtu...
> Personal use is when you install the product on one or more PCs yourself and you make use of it (or even your friend, sister and grandmother). It doesn't matter whether you just use it for fun or run your multi-million euro business with it. Also, if you install it on your work PC at some large company, this is still personal use. However, if you are an administrator and want to deploy it to the 500 desktops in your company, this would no longer qualify as personal use.
Current license with the "noncommercial" requirement: https://www.virtualbox.org/wiki/VirtualBox_PUEL