While it may appear that an article is available for free, it is possible that the article has been pirated or is being shared without copyright permission. To ensure that the open access is not violating any copyright issues, in sources where we aren't sure of the OA license, we want to do some double checking.
All we are doing on Wikipedia is providing a link to the source, so we are not reusing the information (and certainly not selling it). However, we cannot know what the site that has the article is doing with it. The article may be there today and gone tomorrow when someone challenges its copyright. Or, in some cases, publishers will do a kind of “peek-a-boo” with an article: allowing OA for a time, and then moving the article behind a paywall (see ). We want to avoid these problems as much as possible.
A couple of sites are known as "pirating sites" and have been sued for their providing of copyrighted articles. You may come across two of these: the first is SciHub; the second is Library Genesis. It’s a good idea to completely avoid linking to these sites or others like them.
Many Open Access publishers use Creative Commons licences to ensure that the content of the articles published in their journals and on their websites are available to all. Most OA publishers try to extend that license beyond to also allowing reproduction, abstracting, copying (with correct citation) using in a 'mash up' with other material to produce new information, text-mining and data-mining. But, this is not always the case.
There are many other types of copyright choices as well. Some journals (like BMJ, for example) that are not OA will give their authors rights to their work that allow them to publish this work on their own web sites. If you locate an article that appears to be closed access on a journal website, look for an author website that might have the article available as OA.
And, just because some articles are OA from a publisher doesn't mean they all are. Some publishers have a mixed publishing model.