This kind of reminds me of certain sections of the film score fandom, who are always ripping on a certain James Horner guy because of the similarity of some of his music to classical or other film pieces. As someone who has written both music and stories myself (for high school, admittedly, nothing published), I know how easy it is to subconsciously or even consciously take themes, feelings or even a few entire sentences or passages from another work, just because they're so beautiful. I remember one short story I wrote for English class which liberally quoted from popular song lyrics now and then, and I got an A+ for it. :) So, I'm not so hard on other artists who do this, I guess.
Why some fans make such inane and spiteful comments is anyone's guess...Some anime fans seem to share this dogmaticness with the film score fans I mentioned above. One man said it's because the film score fans feel "marginalized" in society, and the anime fans may feel so too. (Yes, I know that's no excuse to be a real jerk, but maybe it has an influence.) Maybe they're "lashing out" at the "popular" things in spite. But this needs more deep thought and analysis.