You're being a jerk about it for no reason at all.
There's no ethic to take into account. The whole point of memes being made is to go viral. When someone reposts someone else's meme, they are helping them accomplish that purpose, which means they are doing exactly the right thing. If you have a personal view about how you want to make your own memes, that's fine, but don't expect others to follow your arbitrary rules, and don't make false accusations about ethics violations when they're the ones who are actually doing the right thing. I love when people repost memes, and I wish people did it more often. It prevents good memes (such as this one) from going out of circulation.
Making a false accusation against someone for plagiarism and theft is a very big deal. It's a false accusation. Nowhere is ZSkellington claiming that it is his own work. Without that claim, no plagiarism or theft has taken place.