You have the right to say this while being the lowest ranking employee in the company, not CEO. But when you're CEO, you have other people you're responsible for, you hired them and they depend on you being able to run the business so they can feed their families. Saying "fuck you" to the customer who is bringing you 10% of revenue is not what CEO can really do.

How is this any different from telling your boss to go fuck him/herself? Many small businesses depend on one or two big clients for their livelihood.