the buffer zone of banking instatutions helps sperate banking and retail,
GM's GMAC is their company they like to finance cars through, the only way GM was able to finance for 0% was that they owned the bank that financed the cars. Welll if GM decided to start taking money from GMAC to keep its daily operations alfoat it would be a conflict of interest of GMAC and would possibly lead to legal troubles for GMAC or financial troubles that they are not in control over but yet have no say.
so if WalMart opened its own bank, and walmart went belly up, their own bank would not start to repo and or sell off walmart locations if they would as if they were a normal independent bank, then the WalMart bank would have no money to pay their suppliers, and if no bank is going to take action on those debts they don't get their money, and since it's not classified as a bank in federal terms they are not regulated.
In my view i don't want walmart having it's own bank, it would be too much power for them.
Atleast GMAC only finances most the cars GM sells and a few homes for their workers, not too much of a conflict of interest in my view.