If it's not in writing, then it's not a requirement. Save that "social" bullshit for someone else. You're not supposed to cheat on your lover because of "social" and moral reasons, but you can't be prosecuted for it (outside of the military) for it because it's not written into law. Don't bother having this discussion with me, pal, because you won't win.
adultery is illegal in more places than the military. so if your basic premise is fundamentally wrong, how is it possible that you could win the argument? also, how old are you, btw. I remember you saying once that you were young.
and actually, speaking about laws, IIRC, the reason restaurants can avoid paying the mandated minimum wage is because it is codified that tips would cover the deficit. So yeah, you kind of are legally expected to tip.