2 thoughts. #1 - I feel you are assuming requirements about the optionality of tipping. The gratuity is optional for whatever reason, regardless of social expectations. So in my opinion one can be moral and ethical without disclosing why they don't tip.

#2 - I waited tables, and a very good waiter once told me that he gives every table the same service whether they do or don't tip, and no matter how much they tip. I feel many servers lack this mentality; which leads to more people tipping less. When I waited tables, 10% was an okay tip, 15% was stellar. Now it seems like 20% is assumed and folks have a social signalling competition to tip 25-30%. Doesn't make sense.