Do Americans understand that it's a backwards practice?   Or is it just so culturally ingrained that no-one thinks about it?

You do realize that the majority of Australian restaurant workers (and other retail) aren't treated that well, and that most working holiday visa recipients, international students, and younger people struggling are paid lower than minimum wage because they're paid cash in hand (illegally), or the business only hires youth because it costs them a lot less than normal (legally).

Next time you go to a McDonalds, a sushi place, or a hole in the wall - look at the staff, and wonder if they're being paid full time wages legally or if the business owner is using youth labor.