In a nutshell, here's how it works. Posts on DU are protected by both "fair use" and by the Digital Millennium Copyright Act of 1998 (DMCA), which safeguards DU and forums like it with a mechanism for dealing with copyright infringements.
Under the DMCA, DU is not responsible for copyright infringements that are posted by its users, provided that we act in a timely manner to remove such material if the copyright holder files a legitimate takedown request.
Under "fair use" people are permitted to post excerpts or a reasonable length from copyrighted sources, for the purposes of discussion. However, "reasonable" is in the eye of the beholder. Copyright.gov puts it like this:
Under the fair use doctrine of the U.S. copyright statute, it is permissible to use limited portions of a work including quotes, for purposes such as commentary, criticism, news reporting, and scholarly reports. There are no legal rules permitting the use of a specific number of words, a certain number of musical notes, or percentage of a work. Whether a particular use qualifies as fair use depends on all the circumstances.
So obviously posting an entire copyrighted article on DU would not fall under fair use, but posting limited portions is okay. Therefore we chose four paragraphs as the limit which we recommend to our users as a reasonable amount of content that will a) prevent DU from getting a DMCA takedown request that will force us to immediately remove the post, and/or b) prevent DU users from getting sued by copyright holders for copyright infringement. When we had a significant legal battle with
Righthaven a few years ago because someone posted an excerpt from the Las Vegas Review Journal, it definitely helped that the DUer only posted a four paragraph excerpt.
The answer to your question "is this problematic" essentially depends on the circumstances. If a single individual were to post an entire article, that would be problematic. If two or more individuals were working together to post entire articles on DU without any of them actually posting the full article, that might be problematic. If someone is merely adding an additional paragraph or two from a long article for the purposes of enhancing discussion, that's likely not problematic.
The point is that ultimately the spirit of the DMCA, the fair use doctrine, and DU's four paragraphs guideline, is to allow discussion to happen while simultaneously protecting the rights of copyright holders. That's what we're aiming for by limiting the length of excerpts.
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