The "Free" in Free Realms does not mean "free as in costs no money" but rather "free as in freedom". FR give players an almost unsettling degree of freedom to do whatever you want and go wherever you want and become whatever you want almost instantly as soon as you enter the game. In FR there is none of the usual MMO hinderance to journying to distant cities just because you have not yet killed a few dozen wolves or delivered a few handfuls of mushrooms to attain level X yet. You want to see the next town? Just go there. It kind of feels like the equivilent of going from Goldshire straight to Shatt (well, maybe not quite that far). The point is that the freedom of movement alone without level restrictions is very fresh and natural-feeling to the player.
Coupled with the freedom to be all classes simultaneously if desired, plus the huge number of quest and minigame choices presented from the first minute of the game, the feeling of expansive freedom is overwhelming and cannot be overstated. The fact that the game mostly costs no money to play should not be overlooked of course, but that is not the reason for the game's title.