You mention that a server at a restaurant spends time working for you personally rather than for everyone. In my mind, that is a major distinction because each restaurant patron creates more work for the server. Pirates do not create more work for game publishers or developers. Sometimes they create less revenue (as in pirates who would have otherwise paid for the game), and I do think it's bad when that happens, but I don't think it's very comparable to not tipping in a restaurant.

It's more like seeing a street performer, stopping and observing for a while, but not throwing any money in their hat or instrument case. If you truly enjoyed the performance for a decent length of time and have some cash on you, then it's pretty dickish to not give any money. (Edit: Granted, pirating a game is a more voluntary act than stumbling across a street performer, but it's actually quicker and easier than finding a street performer these days... unless you or your neighbor are street performers.)

Similarly, if I pirate a game and truly enjoyed it for a decent length of time (as opposed to quitting in disgust one-third of the way through), and if I can afford a legitimate copy, then I will feel morally obligated to buy one. But I won't feel bad about having pirated it in the first place. (Of course, I haven't actually pirated anything in a long time thanks to cheap Steam games.)

Edit: If I saw a street performer but had no cash to contribute, I probably wouldn't linger too long. I would think it's a bit silly for me to feel the urge to move along (it's not like my leaving will make the street performer any richer), but I wouldn't want the street performer to think of me as a cheap bastard who has money but won't give it up. Perhaps it's also a bit silly for me to care what the street performer thinks of me.

Now, if there were a big crowd around the street performer, I would feel more comfortable hanging around, enjoying the show, and wishing I had some money to contribute.