Actually, WMT has clarified their $4 plan as covering "up to a 30 day supply." This change was likely due to several sources exposing the absurdity of their initial "30 pill" offering, as the majority of the drugs on the list are taken more than once daily. An innocent oversight on WMT's part? Unlikely.
However, it's a moot point. After Medicare Part-D, there are now very few cash payers - the only group to benefit from this plan.
Had this plan come out two years ago when seniors, politicians and the media were up in arms about drug costs, it would have been a legitimate problem for the chains.
But after Medicare Part-D, 95% of WAG customers now have insurance in which they pay the same copay wherever they go. So taking price out of the equation, what's left? Convenience.