oa.mg appears to be some sort of search aggregator that wants you to buy the article for $17 even though it's open access (freely available).
https://news.ycombinator.com/item?id=32643396
Repackaging and selling free stuff is a business model that is hundreds of years old.
Occasionally, it's worth it, as the repackaging may come with some help understanding the material, or help in organizing it.
https://news.ycombinator.com/submitted?id=sgfgross
And I can sell you any GPL program I got for free, if you're willing to buy it...
https://news.ycombinator.com/item?id=32643396