Walmart is not the reason a large portion of their income is from food stamps. The reason is this administration thinks the more people they have on food stamps the better and are encouraging many people to get food stamps that would not have previously been accepting them. There is an actual campaign going on to get people on disability, food stamps, medicaid, etc., etc. Walmart happens to sell groceries so they do benefit from all those on food stamps, but would also make money if the economy was better and few were using food stamps.
Except that Walmart has lobbyists looking to funnel more customers their way (they are not along). They also have identified people getting off food stamps as a large risk for their business model. Walmart found out that there was room at the trough, and stepped right up.
Again, many companies do this. Heck, most large companies use the Gov to either get customers or limit competition. But a lot of people doing it doesn't make it right.