Why all these different areas? There is no formula or rule book, only custom.

You don't have to tip anyone if you don't want, but the system here in the US is that businesses expect their customers to tip for service accordingly. Gratuities are certainly not a part of your purchase. Furthermore, I know for a fact that some managers at hotels and restaurants will take staff's side if the customer fails to tip properly, and they later complain about something.

When I was a poor student, and wanted to go out on Saturday night, I would count tips as part of my costs for that night. If I didn't have enough to tip, I wouldn't go out.

It's simply not fair to not tip when you should, and I feel that most of the time, it's your burden to find out when to tip, and how much.