I don't see a problem with using other people's content as long as the Steemer makes it clear it is not his creation; this mostly applies to news and political content, but memes can be shared as well.
memes are meant to be shared!
It can be iffy in profiting from that...
It gets more into plagiarism the more artistic the work; I mean it is cool to share things that excite you and interest you, but it gets MORE than a little iffy in profiting from that.
Finally, when I do reSteem my older work, I basically cut/paste the old post, and then reformat the Markdown code