@real brisker
Just because something is optional doesn’t mean there is no chillul hashem if you don’t do it!
What if there is an old lady in a wheelchair who is right behind you and you don’t hold open a heavy door for her and let it slam in her face, there is no law saying that you have to. so therefore it’s not a chillul hashem?! There is no law obligating one to say thank you when one does him a favor. Does that mean that there is no chillul hashem?!
I know people who have worked as waiters and whether or not they are justified, they do think people who don’t tip are jerks.
I happen think they are justified because it is taken into account when they get wages. You say-well its just up to the employer!-but if you know anything about economics, in your world, the employer will just charge you more for your food if he has to pay the delivery boy more and probably more than you would tip.