I think the BBB gives preferential treatment to companies that are members.
On their website I've seen non-member companies that have maybe 5 complaints and the BBB gives them a B or C rating. And then I see BBB member companies in the same industry with like 25 complaints, many unresolved, and they have A ratings.
With that said, they did help resolve an issue I had with a retail furniture company.
So they are helpful in resolving disputes/complaints, but I'm a little suspicious of their ratings.