I've seen the mods write before that they censor memes that they believe are likely to result in flame wars. They do this for the sake of peace on the site. I understand that it's frustrating to the user.
But the point is, they have whatever right they want. They don't have to explain their standards to us, or go by standards we think they should. Their site, their standards. There's nothing on the site that says all our memes are guaranteed to be featured. There's nothing unfair about it.