I'm with you on this line of thinking. Just a little while ago I made a comment about how I feel we can accept post rewards for linking content that is not our own... but it's a fine line and your post has led me to ponder it more.
The first key is properly crediting, and not claiming the work as your own. I think everyone is on board with that.
The more borderline part is the thought of profiting off someone else's content. In a very basic sense, I think that authoring a post that references non Steemit content is essentially curating, and can be rewarded. If the content in question does not have a Steem presence, rather than upvoting or resteeming, we share it as a new top level item.
With YouTube videos, it's easy. The embedded material still plays through YouTube, and is appropriately monetized and rewarded through their own system. Other content like an article, or piece of artwork, we need an etiquette for.
In those cases I feel in addition to being clearly attributed, we should provide a link to an official site for the content in question. The content also should not be reproduced in its entirety. If someone wants to see more than a thumbnail of the artwork, or read an article in its entirety, posts should be constructed to drive that traffic to the content creator.
If a creator has a Steem presence, good etiquette would indicate we only upvote, resteem, or link to their content.
This would even be good marketing. If a creator sees traffic hitting their site from Steemit.com, they'll check it out, and potentially build an official presence themselves.
The posts you've pointed out are definitely spammy, and Rapidgator is not a content creator!
With decentralization comes the burden on us to police such things. I have no reason to believe this poster (or any other) is malicious, and if they see such content is not favored, they'll stop posting it because it is not beneficial to them.