Windows is mostly sold as OEM on new computers. For existing/old computers they have minimal sales... so they are giving it for free here...

Gut since they will start with subscriptions very soon I wonder what is their long term plan with these Win10 installations on existing computers...

It's only free if you have a valid Windows 7 or 8 license that you have already paid for so really, it's not free at all.

Curious what sort of Windows license hasn't already been paid for.

Well, I bout my workstation without it so it's not free for me. This is really a big scam by Microsoft, for previous PC's I had to pay this Windows fee even when it was of no use for me.