BBB and Customers have very different grades on what makes a good company. BBB looks at upper management, business initiatives, structure, and law compliance while customers look at Service and ground-level employees to determine a companies worth.
Most, if not all, complaints come from poor customer service, unstocked shelves, and people generally not giving a crap as they are being paid dirt and are very likely unincentivised to do better or be better.
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Would you like fries with th... I mean, your milk in a bag?
You also have to take into account the whole "bad news spreads faster than good news" thing. For the most part, people only have something to say when they're upset, not when they had an average-to-good experience that they expect before walking in the door, so good reviews aren't going to be posted as often.