I'm not a legal expert on it but I did spend years being told what was and wasn't plagiarism and had to take several classes on it in university.

To your question: I don't think people give up their rights to their intellectual property, even when it's meant to go viral, I think in our present age it just happens.isn't questioned. By this I mean, people just don't understand the legalities of intellectual property and may believe they've given up those rights. Some may not even know intellectual property is a concept. Some may give up those rights without explicitly stating it.

On the other hand, some get very upset about it. When my books are pirated I'm very upset and try to stop it. When my pictures were downloaded and used in other places on Photobucket I was very, very upset about it. I understand intellectual property though. What I didn't understand was it can happen. That doesn't mean I think it should though.

In my opinion, and it is only my opinion, using someone's property without a commercial license, or their consent, is an act of plagiarizing. If you've made memes with your own pictures then they are yours. If you decide to let people use them, that's up to you. In other cases, the creator may say "here's your lawsuit, now pay up." From now on, if you're worried about it, sign your work, it is your intellectual property. Even if you give up your rights to it, it's still a way to promote your work.

If you're worried about future use of other memes, I'd say try to find a way to attribute the meme to at least a website. Steemit is trying to create original quality content, memes are an iffy subject, as far as I'm concerned. We've seen articles being ripped off, photographers having their pictures stolen, and chefs having their hard work stolen by some shady people. THAT has to stop.

My motto has always been, if in doubt, find something else. I hope that helps in some way, I've spent the day making my brain work on author stuff so I may not have answered you as you expected or wanted, but I'm tired. If you have more questions, ask away. :) I do hope I helped somehow, though. :)