Finding poetry in all the wrong places

So here's a doozy for you: is "found" poetry plagiarized? Andrew Motion's recently published poem, An Equal Voice, is made up of quotations from several generations of shell-shocked soldiers. But Ben Shepard says that almost all of the quotations were taken from his book.

Shepard didn't write the quotes, but he did collect them (although, since they're from a number of generations, I don't think he conducted the interviews himself). Alison Flood wrote in the Guardian article:

Motion said his poem drew on "a long and honourable tradition" of "found" poetry, pointing to TS Eliot's The Waste Land, Ruth Padel's poetic biography of Charles Darwin and Anthony Thwaite's Victorian Voices. "It goes right back to Shakespeare," he said. "It's very well established."

So: plagiarized or not? And, if not plagiarized, ethically questionable?