1. Poetry seems like a medium that is inherently difficult not to plagiarize. Unlike regular prose or journalism, you don't have the room to do gracefully do attribution (e.g. "According to the Times, two roads diverged in a yellow wood") and the shortness of the phrase necessarily reduces the space you have to make something unique.
2. It's great to be involved in an intellectual pursuit, like open source software, where copying and remixing is not only encouraged, but is a best practice.
3. It says a lot about the ephemeral state and nature of poetry that someone could plagiarize Emily Dickinson and Sylvia Plath, in this age of Google, and almost get away with it.
edit: poor wording on my part. I meant more that it's difficult to avoid unintentionally plagiarizing, in that the form of the medium seems to make it difficult to show that you were trying to "remix" (speaking from a layperson's perspective). The examples given in the OP seem to obviously be ripping off, though.
But yes, it also seems from a layperson's perspective that there would be collisions in how things could be expressed...but I'm thinking as a layperson who doesn't appreciate all the ways that poetry can break common rules of language and expression.