WalMart is working with SolarCity in it's "green energy" scheme. I certainly don't know the details of the WM/SC deal, but commonly, SC sells and installs their panels, but retain the subsidies that go with them.
Just a guess, but in reality SC is more motivated by the subsidies than saving America's energy future.
We were in a Wal Mart last week after 10PM. The lights were so dim we had trouble reading labels. Being a suspicious sole, I asked a mini - manager if WM was kissing Obama's ass by "going green".
He said they were working on the lights. We aren't in the store hat late very often. But I will be checking.