Both Publishers Weekly and Publisher's Lunch are reporting that Amazon is disabling the buy button for POD publishers who are working with Lightning Source, rather than Amazon's own BookSurge.
BookSurge, more expensive than Lightning Source, has been trying to reduce the giant market share of Lightning Source. Amazon's "Use BookSurge" mandate apparently extends to traditional publishers, who often use POD to fulfill older titles and produce extra copies when there is extra demand.
Other respected news sources, including the Wall Street Journal, confirm the story.