this is something that probably isn't clear to people who don't live in the US -- wait staff in the US make around $2-3/hour. they aren't paid a living wage. they rely on tips as their primary source of income, and tips are usually shared with a bartender/busboy.
if a restaurant runs a groupon and all grouponers tip on the post-coupon rate, the wait staff will be taking a rather large pay cut for the length of time that people are grouponing. i know i'd be grumbling if something my company did resulted in me making 50% less for a month or two.
more expensive restaurants and meals mean more expensive tips because the wait staff is more experienced, better trained, higher quality, and do more work. one person at a higher-end restaurant can give high quality service to fewer people than one person at a low-end place, and meals at a higher-end place usually take much longer. yes, the average tip is larger, but the frequency in which they receive tips is lower. yes, they make more, but not that much more.