Quite simply, they are trying to have their cake and eat it too here.
People spend more money with credit cards then they do without, thus the business gets additional revenue thanks to having credit payments available. This typically makes up the difference lost from the percentage of every purchase that is taken by the CC company. This is a perfectly fair arrangement IMO since people can spend money at your store that they do not have on them, or currently do not have period...and you get paid for real. The cc company has to pick up the liability on the other end.
I'd report their ass, its quite simply bullsh|t and if linebackers post is any indication, its also 100% against the merchant rules.