So yes, if a series of customers stiffs them, they're allowed to whine, because the IRS is going to go after them for what they should be making.
Part of 'owning it' as a top paying job is paying the taxes you owe for a certain tax band.
Then that is an issue for which the IRS needs to be challenged. I presume all waiting staff are clamouring to notify the IRS when a celebrity/very wealthy person gives them a huge tip? It can't be that unusual for some people - whether well-off, very satisfied or (I suspect more likely) tourists who don't really understand it but who are frightened of accidentally causing offence - to give considerably more than the expected tip. Do waiting staff run after somebody to tell them that they have overtipped and offer them some money back? I think I can guess on that one.
You keep talking about customers 'stiffing' them, but how can this be in an industry (which I accept isn't designed that way by any one individual worker) that states a price and then expects people - including foreigners - to know that they're expected to pay $X/X% more than the supposed advertised price? Either you're genuinely grateful for extra money that people give you by way of gratitude; or it's part of the price, in which case it should be stated as such.
It's always a gamble when you agree to sell something but understate and don't confirm the price you're wanting for it in advance, in the hope of getting more money overall: many you will win, but some you will lose.
The anglocentrism is pretty strong there. 'America is so weirdly and determinedly backwards...'?? Or maybe just different?
I agree with you that it is very unfair and completely unhelpful to slur an entire nation based on one cultural practice that some foreigners find bewildering; but it works both ways - 'somebody' upthread effectively slurred all Brits by claiming we don't 'do' or understand being nice or kind to each other.