In defense of the indefensible, I have also had the experience of being in a large party where the server “knew” she was getting an automatic gratuity, so she was less than diligent with her service. We had to call the manager and have the gratuity removed from the bill.
But that is no excuse for theft.
I don’t mind tipping, because you have the option to tip less or even nothing for poor service.
I was trained to always tip 20 per cent for good service; I can only remember once when I received really poor service.