NPR has the story. Under federal law, of course, RPM is subject to the Rule of Reason. Apparently the Senate is interested because a large portion of the contact lens market is subject to the restrictions.
It is unclear to me whether the manufacturers have truly nationwide policies or whether they have excepted those states that still treat — or may treat — minimum RPM as per se unlawful.
Update: After hearing a bit more about this, it sounds like these are Colgate unilateral pricing policies (no agreement; if retailers don’t abide, they don’t receive more product). But I’m not entirely sure.