The really big bookmakers have a lot of customers. I don't think they are easily steered by the fact that they have a problem with a customer. After all, every bookmaker or casino has a problem with a customer, often related to payout of course. But it's true, a reputation can be destroyed in moments.
You underestimate the power of negative reviews. Every good businessman should care about EACH customer to maintain a good reputation and be ahead of constantly emerging competitors. Imagine, a certain online casino has 100 positive reviews and no negatives, that gives them, say 5/5 stars. At this point there's nothing that can stop potential customers from using this casino. Then, one day, a very bad review appears, which tells a horrible story where someone won $5k at the casino and couldn't withdraw the win. People will pay attention even to that one single neg, despite all the positives. As a result, due to mistreating just one customer a casino can lose many potential ones. At the same time, the other casino which has no negative (or less, because none is, probably, impossible) will win over the customers.