In my opinion, it is definitely not opinion-based and I voted to leave it open when these close-votes were cast.
So, why was it closed? Most likely people agreed with this comment by @Krazer:
There is no right answer to this. Call it what you will. The fact remains Anime == Cartoons. In Japan all cartoons are anime. In Korea, aeni, in China, donghua. People call it by the word from their respective nation because it distinguishes. It's what makes a Deep-Dish pizza different form a Sicilian pizza. Both are pizza, but quite different from one another. There is a designation, Avatar is simply an anime-style of animation. If I made a Turkish lamb hamburger with walnut-based sauce. All the spices and ingredients might taste Turkish, but you know a hamburger is not at all Turkish.
However, I think that this doesn't make the question opinion-based, and in fact is where the answer lies. Just because we cannot say objectively that to all people, all the time, Avatar (is or isn't) an anime, doesn't mean there is no answer. It just means the answer is slightly more elaborate than yes or no, which is totally okay.
At least twenty-one people agreed that the answer given was good enough to be worthy of being posted, and answered the question in a helpful manner. To me, that's reason enough to say that it's as free of bias, opinion, and influence as we can hope for on a site like this.