If a poet’s work is plagiarized and republished without consent, it not only devalues the poem itself, but also undermines the writer’s vocation and livelihood.
I would argue that even if a poet is an amateur who doesn't have a "vocation and livelihood" that is dependent upon his or her poetry, plagiarism still devalues the poem. A poem comes from the writer's individual and unique experiences and viewpoint, and taking someone else's words--particularly with verbatim copying--co-opts those experience and that viewpoint.
On the Internet, content is king: a potential offender might plagiarize a poem not for its quality but rather to compile it with similarly themed poems and in the end profit from ad revenue based on a keyword or subject.I think in the cases of amateur poets posting plagiarized poems on poetry web sites, blogs, message boards the motive is not so much profit from ad revenue as it is adulation from other amateur poets. A writer friend of mine once said that there is a hidden (and sometimes not so hidden) egotist in every artist. We want to communicate with others but we also want to be liked and want our work to be liked and appreciated, whether we admit that publicly or not. That sentiment has stuck with me over the years, and I have come to appreciate the inherent truth of it.