I think for the most part, people who complain about reposts aren't sincere. They don't actually see any problem with reposting. Most of them have no problem doing that themselves.
I think they're just frustrated/envious that the meme in question is doing better than theirs. However, it doesn't sound good to write "I'm jealous that your meme is doing better than mine," so they just write "Repost" (sometimes without even checking whether that's true).