In response to the story of the ESRB's call for the removal of the Dark Sector trailer due to its "excessive or offensive content", Destructoid got in touch with them for an explanation.
Here is what ESRB president Patricia Vance wrote.
"The ESRB's Advertising Review Council (ARC) regularly monitors game ads and trailers to make sure that they adhere to industry-adopted Principles and Guidelines for Responsible Advertising Practices ( http://www.esrb.org/ratings/principles_guidlines.jsp), which were established in 2000. Since 2005, ARC guidelines have required that trailers for M-rated games on publisher websites be displayed behind an age gate to help restrict viewing to those visitors who are 17 and older. Game publishers are also required to use best efforts with respect to ensuring the presence of age gates on third party websites that display their M-rated game trailers. If a third party site insists on carrying a trailer for an M-rated game without placing it behind an age gate, our guidelines require the publisher to request that such trailer be removed and/or provide an edited version of the trailer to be used in its place.
However, the mere presence of an age gate does not permit a publisher to simply put whatever content it wishes into the trailer. All trailers must still conform to ARC's Principles and Guidelines, which prohibit the display of excessively violent content or any content likely to cause serious offense to the average consumer. When ESRB notifies a publisher that the content in a trailer is in violation of these ARC requirements, or that there is an age gate issue on a third party site, that publisher then must notify third party sites to rectify the problem. The notices issued recently by game publishers to third party websites are simply that - steps in a chain of publisher compliance with ARC guidelines and the ESRB enforcement system that have been occurring since their establishment seven years ago."