dMania was from the beginning a platform for sharing content. Some users called that plagiarism or stealing content. [...] So on a platform like dMania you are rewarded for sharing the content, not necessarily creating the content. [...]

Look when somebody takes another persons work without their permission that in itself is stealing when it comes to the physical world. Now of course the definitions can be a bit more loose once you enter the digital space but the moment you start making money from the work of somebody else then you are being dishonest by just calling it "sharing".

Some would say it makes a difference because on Steem you get paid and you benefit from the work of somebody else by posting it on dMania.

Yes, exactly.

What they don't understand is that somebody already was benefiting, the big companies that run platforms like 9GAG, Imgur or Reddit. Every single top posts on those platforms is worth thousands of dollar to them. The difference to dMania is that those posts now have a price tag and the users get rewarded.

Everyone understands that those companies are making money on the content that users post. I presume that you claim that we don't because you, for some reason, find it less moral for a company to make money from stolen work than an individual. That does not matter. The immorality of an action is not determined by the identity of the person who performed it. If you are going to claim that it is immoral or wrong for a company to make money from stolen content then it is no more moral for a regular user that "shared" the content to make money from it. It's as simple as that. If 80% of the content has been pulled from somewhere else then it still comes down to 80% of the profit coming from stolen content. Granted it is 20% better but that doesn't fundamentally change the core of the issue.

I strongly believe in a community were sharing is everything. A meme is like a joke in form of a picture. For a joke it doesn't matter who created it, it is only important that it is shared and brings joy to people.

This does not hold weight even with something as pure as a joke. You should probably listen to professional comedians talk about this issue. They care about other comedians stealing their content and they don't let them get away with it as long as somebody laughs. Both for monetary reasons (it is their living - they do this professionally) as well as personal pride in their creation. Some people might not care but a lot would. As soon as you introduce money into the mix people will get upset about others profiting from their work. I am pretty sure you can replace money with whatever other form of rewards and the result would be the same.