10.1 was also free for 10.0 users, though mostly because 10.0 wasn't really finished (some would argue 10.1 was not either)

OS X is a seperate OS. Apple didn't say that this is the first time they had given away an OS. They said this is the first free OS X release. Which it is.

"Today we're going to revolutionize pricing. Today we announce a new era for the Mac. Today we announce that Mavericks is free."

This is a quote from the Verge's liveblog of an Apple guy. Note he said "Mac", which made me think of it.