Profit margin is not the same as profit. The margin is the difference between how much it costs you to run a business, and how much that business makes. So for example, a restaurant can be full, but if they're only going to keep 10% of the money being spent while the rest goes to food, rent, exployees etc, then its still a business thats barely getting by, regardless of how busy. Restaurant margins are higher than 10% I think, but they are notoriously thin and hard to manage.
Also over 50% of restaurants fail in the first three years (the number was thought to be closer to 90% but new research has lowered the number -
http://researchnews.osu.edu/archive/restfail.htm)
Let's say they have four waiters going per shift, 2 shifts a day. If its 6 hour shifts, thats a total of 72 hours of employee time. If they had to pay them 8 dollars an hour instead of 2, that would be an additonal five hundred dollars a DAY. Most restaurants do not have room for another $3500/week in their budget, whether they're packed or not.
And that's a small place with four waiters. The bigger and more successful your place, the higher that number will be. A really high end restaurant with higher prices is probably already paying their waiters more, because they need to be selective. But now they've probably got 150 employee hours a day so theyre looking at more like 8 grand a week extra, maybe more because the waiters tips would be higher in a place like that, so the restaurant would have more that they had to compensate for. Times 52 weeks is 400,000 dollars a year. I doubt many high end restaurants are so profitable they could throw four hundred thousand dollars away.