Just because it was free upgrade, it does not mean, that it was OS for free.
You needed a licensed Windows 7 or 8 in order to upgrade; that one costs money too. You weren't able to get entirely free upgrade. It was more of exchange.
I bought it through the Window's store after I bought one of the first Dell XPS 15 Windows 10 models. The only reason I needed pro was to enable HyperV to use with Vagrant.
You needed a licensed Windows 7 or 8 in order to upgrade; that one costs money too. You weren't able to get entirely free upgrade. It was more of exchange.