Beyond what you have written the entire notion of "intellectual property" and its associated protection mechanisms is a quagmire of contradictions. That's why we have lawyers making careers sorting it out.
It seems said notion is WHOLLY about money. Remove the possibility of profit and the problem evaporates. No one actually cares about the intellectual property; they care about the money it might generate. (OK, there might be 3-4 that care about attribution too owing to their vanity)
So, if we accept the above statement as correct we can formulate a behavioural guide for ourselves, a usage litmus test if you will. Simply ask the question
Will my usage of the other person's content increase or decrease their POTENTIAL revenue from their content?
If our usage INCREASES their profit potential then they should be happy. If our usage DECREASES their profit potential then we should NOT use their content.
(One obvious exception is when content is labelled as COPYRIGHTED. In that case they creator has stated they want NO help from others.)
On a medium like the Internet, eyes on content equals money so in most cases simply attribution with links to the source should be seen as increasing potential profit, and minimally as free advertising.
While not a perfect solution it at least causes us to pause and should afford a workable solution in most cases. But plagiarism has never been a problem among us who actually care. The problem comes from those who have no regard for anyone but themselves.
A problem with this unfortunately is that you don't know if the original creator is also on Steemit. I had a video which no-one should have made money out of (it was a music vidoe where I made the visuals and a band made the music and it was made for tertiary study and I signed an agreement saying I could ptu the video up on YT etc but couldn't make money from it). I shared it on reddit then came here to make a post with rewards turned off while I was at it and some turd shared my content off of reddit in the time it took me to make a post and started getting upvotes on it. His post got all the attention and mine got hardly noticed, even thoguh I was the original cc. I managed to get the rewards turned off on his post, but this brings up an issue of "what if I could have made money from it and didn't have that agreement?". In that case using the logic you stated in your comment, if you assumed I wouldn't have been on Steemit, you would say the free advertising would have lead to more profit and shared it but because I was on Steemit, I did make a post but your post got upvotes and mine didn't, which would mean I lost profit from it rather than gained (in the case of the real situation, the freeloader's post did gte more attention than mine did, but luckily I made it so neither of us made money from it since that isn't allowed). I have no issue with people sharing with rewards turned off but they shouldn't be making money from content they didn't create. And unfortunately we don't know for certain if someone is on Steemit or not either.