The flaw in your logic is that sharing something does not equate to compulsory monetary compensation. There is a difference between a sale and sharing. I'm not necessarily talking about plagiarism.
If I share my lunch with you, do I ask you to pay for half? Maybe. But that's a contract agreed upon by both parties, verbal, informal, or otherwise.
Yeah, sharing is great, but I don't agree that people should be financially compensated for sharing work that isn't created by themselves. At least not without having explicit permission from the original content creator.
For example, you can promote this practice on Steemit by only upvoting posts where the author is also the artist or creator of that content. If there's great content already made and you want to share it, resteem it.
The fact that people are abusing Steemit in this way already is irrelevant to my initial point: Steemit already has a mechanism in place to devalue posts like this, which is an incentive for people to use it. Whether or not people do, is another conversation.
To rephrase my initial point in the form of a question: How does creating a whole platform for people to make money off of memes benefit society? What kind of a culture are you promoting? Even if you don't know who made the meme, what gives you the right to make money off of it instead?
...If the point is just to have fun as you say, then leave it on 9Gag and let everyone have fun - that's not the problem here. The problem is that the second you introduce monetary (or other) compensation for fun that relies on other people's work it becomes a whole new ball game and is asking for trouble in the long run.
I can't stop the internet obviously (and nor do I want to) - I'm only bringing up this argument for the sake of food for thought.