Yes, racking my brain trying to figure out how a tip is a must in the USA..
Unless there's some European humor involved that I'm not aware of 
In the US, you are expected to tip everyone for everything. If someone holds the door open for you, they are probably expecting a tip (exaggeration, I know, but you get the point). Everywhere else, tips are appreciated, but you won't get angry looks or even rebukes for not tipping...