[Update] Microsoft and Machinima have released a joint statement addressing the YouTube controversy:
This partnership between Machinima and Microsoft was a typical marketing partnership to promote Xbox One in December. The Xbox team does not review any specific content or provide feedback on content. Any confidentiality provisions, terms or other guidelines are standard documents provided by Machinima. For clarity, confidentiality relates to the agreements themselves, not the existence of the promotion.
So by the looks of it, the promotion itself wasn't what was intended to be kept confidential, but rather the terms of the partnerships. In addition, it appears Microsoft didn't specifically impose terms on users to provide only positive coverage.
[Original] Microsoft has allegedly made a deal with Machinima partners to get positive Xbox One coverage on YouTube.
According to a report from Ars Technica, the Big M wanted to keep the details of the agreement confidential, which clearly is no longer the case. iGame Responsibly has a screenshot of an alleged email that was sent out to YouTube users. Check it out below.
Not only that, but Machinima's UK community manager posted a message on Twitter about the partnership deal, which has since been deleted.
Furthermore, Arts Technica goes on to point out that this may go against FTC regulations, which state full disclosure is required when "a connection between the endorser and the seller of the advertised product that might materially affect the weight or credibility of the endorsement."