The point here is that Walmart left, essentially, nothing to its employees’ discretion. It didn’t trust them to do the right thing: it codified everything in a set of rules, and then told them to follow those rules. And you can see how that might have resulted in a kind of Calvinist scale-blindness, where accepting a soda when going to meet a vendor is exactly as bad as greasing Mexican wheels to the tune of 24 million dollars.
On top of that, the most senior executives at Walmart had a lot of discretion when it came to enforcing the rules. For someone like Roehm, who never fit in to the corporate culture, it was easy to find an infraction and fire her. On the other hand, when it came to allegations touching on Castro-Wright himself, it was similarly easy to hand the investigation off to one of his loyal subordinates, who did what he was expected to do and buried it.