That’s an expected tradeoff of operating an essential service is the point. It’s not as though their margin is that slim by mistake, or out of goodwill, or bad business sense. It’s meant to lead to the situation where we shop at Walmart not by choice, but in lieu of other options.
I’m really meaning the lack of option not to consume fast-moving consumer goods, rather than the option to pay a premium for them elsewhere. When their market position is similar to like an outlet for government rations except for private profit, their net is essentially what was skimmed off the top of free enterprise. 2.66% is just the current maximum amount that is justifiably worth without doing societal harm