Well, I wouldn't go as far as to say it was "stealing". Plagiarism maybe yes, but not out right stealing.
I mean, usually, the article is placed in the "Public Domain" such as on a free article site.
In that case, nobody holds any legal rights to that article and no license or usage rights have been attached to it.
Also, once its been modified (spun), each new spin output is effectively a new (different) article to the first (original) article.
99% of the articles on the Internet, on blogs etc, are made up of rewritten, rehashed articles that someone else wrote.
Where else do you get ideas for your content?
Sometimes I visit article sites, find an article and completely rewrite it in my own words.
I'll literally have notepad open next to it on my screen and rewrite that sucker in my own words.
I can also expand on that article then too, add my own views/take and maybe make it an even better article in fact by including more new things about whatever subject its on.
That's article rewriting though not spinning I know.
As for spinning, even if you take someone's old article and spin it, yes that is basically just plagiarism and very lazy and sloppy too.
That said, some people do get away with doing this and posting it to their sites.
But you have to be careful because machine spinning content automatically usually creates very poorly spun articles that no longer even make sense grammatically etc.
The best way to spin an article is by doing it manually by hand, one word, phrase, sentence at a time.
It takes longer but this can produce much better spins that make sense on each output. Idioms are correct etc.
But even then, I would never use a spun article on my own site!
I maybe would use it for creating Tier3, Tier4 content links to my Tier2's etc but never directly to my money site.
And if I do spin my articles, I write my own unique article anyway, I never scrape other peoples articles and spin them by hand.
That is some noob level, 3rd world country crap right there!
Don't cut corners, just do a proper job.
Using machine spun content that you scraped and plagiarised will come back to bite you eventually.
But again, as said, some people do get away with it. It depends how you go about it.