There's seems to be a lot of it going about, or at least I've seen a handful of complaints about it. Why you would want to do this, I've no clue. If you can't write the piece on your own then why claim another's work as yours? What's the point in doing it? If you think that you won't eventually get caught you're delusional. It might take a great deal of time, but eventually, it will happen and then you'll simply be a liar. So again, why do it in the first place?

It's something I don't understand.

I wouldn't want to be applauded for work someone else did. I want the little gold star for work I accomplished. And if I discovered that someone was posting work of mine without my permission you bet there would be holy hell to pay. I don't care what context it was in, YOU DO NOT DO THAT! 

Whether you're passing it off as your own or, as I've witnessed a couple of people doing, making what they call "appreciation" accounts. You do not repost another's work without their permission. It's simply wrong. If you want to appreciate a writer's work make a book listing the titles of their work and dedicate the chapter to them so others can go to their account and read their work. There is NO reason for YOU to post another person's work. NONE. EVER. 

pla·gia·rism  [pley-juh-riz-uhm, -jee-uh-riz-]  noun

1.       An act or instance of using or closely imitating the language and thoughts of another author without authorization and the representation of that author's work as one's own, as by not crediting the original author

2.       A piece of writing or other work reflecting such unauthorized use or imitation