As an English person, in the UK by not leaving a tip, I am stating that the service was adequate, and not outstanding. If I complain, then the service is REALLY bad. If I PAY someone, it's because they've done something really personally wonderful.

I fail to see how it is my problem that waiting staff get screwed by their employer if I don't leave what I consider to be a completely optional service charge. If the employee has a grievance, then it should be with the employer, not me, and to accuse ME of stealing? sorry, what? I don't pay an optional fee, therefore I steal. How is that in the least bit logical in any other industry?
We, like most of the civilised world, have a minimum wage to ensure that workers are not slaves. If the US treats "tipping jobs" differently, then that is not my problem. What is worse, it is expected that I reward (pay) for someone if they provide a poor service. You may want me to cause a fuss to management, but when I pay for a meal, I expect a meal, in a reasonable state, in a reasonable time. nothing more, nothing less. I'm not paying to have an argument. I also fail to see why serving me an expensive dish is worth more to the person serving me than a cheap one. It's not like they're doing more work, is it? a plate is a plate, regardless of whether it has caviar-stuffed lobster or a big-mac on it. It's not my job to employ someone for a service when I've already paid for it. That's the restaurant's job.

If the US system is set up in a manner which requires irrational behaviour to function, "because that's the way it's always done", then that's not my problem. The only other times that "optional" charges are actually mandatory, are where you are involved in corruption and bribery to get things done.

All of the above said, when in the US, I pay the 15/17/18/20% is to avoid confrontation, and partially because the server is caught in the system and relatively powerless (but that doesn't stop them lobbying for a law change in a free country, does it), but that doesn't prevent me from begrudging the system in the first place. Besides, it gives me something to complain about in the pub when I get home