You've hit a few proverbial nails on the head for me there. Having lived primarily in a number of countries where tipping was entirely optional and rarely more than 10%, I still find it awkward when I travel to the US. I generally follow the 20% lead these days but the figure is always determined based off the service I've received, as I've justified the absurdity of it as "the wait staff get ripped off by not being paid a fair wage and I have to top it up for them". There have been occasions where we've dined in quite expensive restaurants in NY and the food has been bad enough for me to send back (something I'll rarely do). But on both occasions the wait staff were polite and understanding and stuck the items off the final bill. I stilled tipped at 20% of the original bill + a little extra because at the end of the day the fact the meal wasn't cooked wasn't their fault and I didn't think they should be punished for it. Sharing that tip with the rest of the staff makes a mockery of the whole process and my justification.

And then I think; In almost every other profession you're expected to do your job to the best of your ability every day. Your "tip" is still being employed the following week.