To me, it's one thing to sample from a completely different genre or style, and repurpose what you've sampled for a completely different kind of work. I mean, if I take a voice sample from Blade Runner and put it in a trance song, it's not so much about ripping off. It's just sampling... quoting, sort of. This seems like less of a problem to me.
If you're an ambient musician creating drone work, and you take a large section of another ambient artist's drone and make it the basis for your own track, that's entirely different. To me that's just laziness, a terrible, lame shortcut... and much more like stealing or at least dishonesty, even if you credit it. You're using the person's work for something very much like the original work, barely changing it, certainly not repurposing it or creating something very new/different from it.
And yes, loop libraries or commercial sample packs are totally different. Those are sold with the express purpose of being used in other people's work.