Why are you saying "In theory"? and "Supposed to"? Wouldn't not making up the difference in these cases be illegal?
What I'm hearing is that if everyone decided not to tip, then servers would(should) still walk away with minimum wage. So servers aren't incentivised to change the system because sometimes they take away more than minimum?
Except the same is true for other tipping cultures, minimum living wage is garunteed, tips are extra.
Seems like the US has the same outcome in terms of meeting minimum wage but with many complications and without the legal protections for the employee.