I think it's perfectly acceptable to refer to RWBY as an anime, for the reasons [@Altered Tundra] and everyone else has mentioned and more. I'd rather call it an anime than a cartoon in any case.

As far as being required to have been made in Japan to be called an anime goes, that's just not true. Anime is a Japanese [b][i]style[/i][/b] of motion picture animation. Saying a show can't be made in a certain style because it was made somewhere else is like saying a Philly cheese steak can only be made in Philadelphia.