You can across the stage free in ranked & arcade mode, as well as play on it if you join a Versus match where the host has purchased the map. Also, they never said the map would be free, they said they're looking into pricing.
I guess that's fair. I can't really complain in the first place. How many game actually come out with FREE DLC. I'm satisfied with just the characters being free. I'd be much more upset if the characters were priced and the stage was free; that's just be a bummer.