It's free for anyone with supported hardware, by the commonly accepted definition of "free".

Stab in the dark, but, I guess it's because it requires the App Store to get it, which, AFAIK, first became available in 10.6.6, meaning, Snow Leopard is the farthest back you go.

If you get someone to DL it for you and burn it to disk/USB drive, you can probably also install it on Macs with older versions, since you'll just boot up in it..

So, it is free for everyone, but, only distributed easily to Snow Leopard and thereafter users.