[CaptainZapp]

You are expected to tip 15% for service, even if mediocre or outright bad.

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.

[blamazon]

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.

[jjeaff]

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.