Many an attorney has gotten rich off of people trying to answer that question!
Taking someone else’s work and simply putting your name on it is plagiarism. No doubt.
That said if you take different ideas from different places / sources (a skill called assimilation) and create something new (synthesis) you’ve moved well up the Bloom’s Taxonomy pyramid. You have applied your own thinking and transformed something (a concept, or in this case, an instantiation of a concept which is the loop / sample, etc.) into that which didn’t exist before.
That’s the academician in me talking. I would also insist on citing proper sources to give credit to those whose works I referenced. Anything less would be claiming their work / research as your own.
Since these companies created and marketed these samples / loops as royalty free for the very purpose you’re asking about I contend they have removed themselves from the “is this stealing” discussion. They’ve waived their rights.
I like what Spitfire has done with the “free” version of their BBC Orchestra samples. They’ve given the option to contribute to a fund from which they pay the musicians whose talents were used to create the samples. I use that library and love it. I also paid the small stipend to make sure those musicians receive something (maybe just enough to buy a Happy Meal) for their time and talent.
Sorry for the diatribe. These are my immediate thoughts. This and twenty five cents will buy you part of a postage stamp.