Well, I don't want people to think that art theft is "reasonable", "too normal to be opposed" or "accepted". There is an explanation of why art theft is considered a crime.
Unlike memes that can be stolen due to not being taken seriously, art theft is a crime in the internet, many art communities, mostly in art websites like DeviantArt. People in the internet, mostly artists despises art thieves, and stealing people's art or plagiarism can lead to a ban. Not just art theft is consider a crime, but it can shatter the spirit of the one who originally did the artwork. This is why most art communities and websites add the rules that says "If you are reposting an artwork that isn't yours, keep in mind that you should credit the artist", "Do not steal somebody else's artwork and claim it as yours" or something like those. Man, even the friendly Digital Art moderators take issues about art theft seriously.
So in conclusion, even though art theft is normal, but it's still a crime and isn't tolerated to the internet, mostly art communities.