Not plagiarism, but downright theft! However, your question is not as straightforward as it may seem.
What about videos of performances (live gigs), posted here first time? Or even carried over from YT orFB? Technically speaking, the blogger makes the content of the blog (filmed, edited, posted) and plenty of their time went into it, including spotting the event, concert etc and attending it. What else does a journalist do? A photographer? Or a channel hosting contestants or a talk show? Then again there are hefty deals involved in broadcasting soccer matches and live performances - may all the sponsorship be factored into that.
So what about the tiny, local performing artists? OR that blogger who might get cited (almost) enrtirely and unwittingly? Can't we put up excerpts of Moby Dick? Analyse the sutras of Patanjali? Joe Bloggs (what's in a name) might benefit from the exposure, but did they give outright permission? Are they getting the RIGHT kind of context? In any case, it makes for a very patient waiting game for anything to get back round to them, if ever they were to get a lucky break ... but that's showbiz, I guess! On the whole, however, I think the uploader is saying something about their life rather than representing themselves as an agent, or exploiting a gold-mine. It only becomes a real concern if they were to earn loads over the back of another deserving content-maker (here on Steemit or not). One would like to think there is this thing called personal honor that kicks in naturally.....and that in a brave new wordl the rest is shared and common intellectual property (and clearly history - never to be retrieved- within 3-7 days). Present replies excepted! (This post deserves a repost! But who knows it may be trending anew today... Are you going to do the research on that....pffff?)
I personally see all of Steemit on a more subtle (energetic) level, and find the same old, same old. I find gross flaws on an inter-connective or community level. Referring to outside sources and using third party material (videos of others) seems to be getting some people into some nasty trouble (flagged, downvoted), while I encourage encorporating anybody else's work (cited, credited) since it broadens out the field and is the best prevention to corruption in-house.
We are trying to be holistic (not to mention sometimes naively idealistic) on Steemit, but then you would, ideally, not mind if your piece was claimed even by somebody else (it could be conceived as a compliment!). Providing you opted in for the ideology of Steemit you would have to trust it would pan out ok. But the money in the mix changes much....
Here, it seems, we are still being "hired" (for the daily paper that Steemit approaches most of all) for who we know, the trending tag we signed up, for or some kind of starlet wow-factor, not paid on an hourly basis nor rewarded for original or intelligent content. The market principle seems bitterly unfair to me and there are too many Steemians who have to suffer the life of a misfortunate African cocoa farmer who is missing the right paperwork (hence overlooked).