Do you know how it is free? The cost of hosting static document only sites will be low but not zero. So is the cost being eaten by the owner or are adverts/tracking inserted?
if you have enough money, it's a common strategy to make a free product to attract users. Once you have an important user-base, your company has a value
So the upsell comes later?
Of course!
not always. i'd be happy to throw a little money at hosting a ton of stuff if it served as a load-test for my work. unfortunately, trying to find something to give away that other people will use isn't as easy as one might think
Even if you don't have a lot of money, you can probably run a niche product like that for little money, as long as it doesn't get too popular.