Honestly as someone who lives in USA I hate this practice. Every time I am mad and want to tip low I think hey they have a family maybe or just had a crappy day this time and I don't want to deny them the ability to pay rent. The idea that my skimping on a $3 tip may be the difference between being able to enjoy a leisurely cup of coffee after a crappy day at work or not makes me tip no less than 20%.

I'd rather they just be paid a livable wage and tips be optional.