The nasty business of doing business.
This fight has been going on for a while, and for anyone interested here is a link to maybe what started it.
http://mobile.reuters.com/article/idUSBREA2Q2BJ20140327
As stated above Walmart is not always the nicest corporation to do business with, and when the American people were being reminded how bad the Communist system was, Walmart was busy buying products from that country, causing a loss of jobs in their own country. I'm not going to say what Walmart does is completely wrong since we do live in a capitalist society, and to be a true capitalist you do business with whoever.
Another company that suffered from Walmarts idea of capitalism was Vlasic pickles, another one of the situations of you do business on our terms or we don't do business.
In a way this action against Visa is likely a move to get even with them, sometimes business is nasty, and in this case the customers who use Visa are being punished.