It's free simply because that's the viable business model right now. "Free to play", also known as "F2P", is a common setup within games these days. You're able to play for free but usually get a large variety of benefits and whatever by paying for various things. In World of Tanks, for instance, you could easily get more hangar space (alot more) by throwing a small bit of money for it, or maybe get a premium tank or two (special versions of certain tanks; their stats werent actually BETTER, but different, fortunately, or everyone would have hated them!). Every F2P game has it's own unique ideas for this stuff.
Anyway, as for me, I'm familiar with World of Tanks but have not played Warships at all. But now that it's on my mind I think I might give it a go, simply since I like WoT as much as I do (really, it's a great game) as it's the same developer with a similar style.
As I understand it these games are particularly popular over in Germany? Well, that's what I"ve heard anyway. WoT in particular is big in some areas. But even in the US where I"m at, I can easily get matches going even in the dead of night; so it's a pretty big game. I expect that Warships is probably similar.
I'll see about trying it today.