It may be normal and a common practice, but none of that implies that such practices should be seen as "normal" or used at all. Ethics cannot be derived from argumentum ad antiquitatem or argumentum ad populum.
I'll make a slightly stronger argument though. You should do business with those you trust. There is no rigorous rational way to determine someone's trustworthiness, so you must get to know them and see how they respond to situations and ultimately rely on your intuition for making decisions.
It's just human nature and I seriously doubt it will ever change. Business is usually built on a relationship of trust, so of course if you know someone you will get better treatment than if you are a stranger.
A good example is a new startup giving an executive a free account to try to get their interest. According to you, it's unethical, but it actually helps small companies compete.
1) common
2) according to social norms
Neither of which are actually the same as ethics.
I'll make a slightly stronger argument though. You should do business with those you trust. There is no rigorous rational way to determine someone's trustworthiness, so you must get to know them and see how they respond to situations and ultimately rely on your intuition for making decisions.