I'm probably tipping way too much, since $1-2 at most airline clubs get a surprised look and profuse thanks. Especially for soda.
The world doesn't need another tipping thread, but part of the problem, at least in the US, is that the tipping is customary based on the server getting less than minimum wage and making it up with tips. But it's culturally taboo to ask every server their financial stats at the conclusion of the transaction.
High-end servers or bartenders where I live easily make over 50k/year in tips. By the minimum wage logic, they should be getting a lot less in tips.