The only case I can think of where there is a mandatory minimum tip in the US is that a lot of restaurants will charge a minimum gratuity for parties larger than 6-8.
To put it explicitly: if you don't tip, you're personally choosing for that person to make as little as $2.13 an hour for that transaction. Most would balk at pulling that lever.
Well, you are actually pushing their wage downward, but not to $2.13. The lowest would be the minimum wage of the area since the company is required by law to make up the difference.
You're expected to tip north of 20% for good service.
Sure, in theory this is not strictly mandatory. Socially, if you don't play the game you're a leeching pariah.