Senator Richard Blumenthal wrote to the FTC Commission Chairman in December arguing that Amazon's policy could "stifle" competition and "artificially inflate prices" for shoppers. While it's not certain that the FTC intended to investigate, Amazon's relaxed approach could address potential criticism by allowing sellers more flexibility in where and how they pitch their goods -- and, of course, lower prices for customers in the process.

The company has faced multiple investigations into the competitiveness of its sales over the years, including Kindle book pricing and its discount policy. Whether or not that history played a role in the policy shift, it likely wasn't keen on drawing more attention to its behavior.