From a Wikimedia Blog Post:
Half a billion people use Wikipedia every month as their source of knowledge. Wikipedia’s community editors work tirelessly at maintaining the accuracy, transparency, and objectivity of the articles, which requires identifying conflicts of interests and removing bias. Editing-for-pay can be a source of such bias, particularly when the edits are promotional in nature, or in the interest of a paying client. The Wikimedia Foundation is committed to continuing to support the Wikipedia community’s efforts to keep articles free of promotional content.
As explained in October of 2013, we believe that undisclosed paid advocacy editing is a black hat practice that can threaten the trust of Wikimedia’s volunteers and readers. We have serious concerns about the way that such editing affects the neutrality and reliability of Wikipedia.
From the Letter:
Several editors raised concerns about the impact of this amendment on good-faith editors, such as first-time editors who aren’t familiar with our rules, or editors who work on projects with GLAM (galleries, libraries, archives and museums) or with universities. We ask everyone to be respectful of others and to help enforce existing community practices and policies about privacy and harassment, even in cases of suspected paid advocacy editing. The amendment is not intended to impact participants in GLAM projects, or professors, when they are writing about topics of general interest on their own, rather than writing about their own institutions while being compensated directly quid pro quo, for example.