After you've been riding with traffic a while, you start to get a feel of the flow of things. It doesn't take long, really, the human brain, being the amazing machine it is, starts to recognize relative speed and direction, and to start predicting where cars are going to be based on where they are.
After a bit more time, you start to time your own moves, like when you're about to make a left turn, you get over to the left side of the lane, look at the line of oncoming cars and slow down a bit, getting your own speed just right to hit the next gap that's big enough for you to safely ride through.
Then somebody comes to a stop to let you turn, and your rhythm goes all to hell, and you nearly fall off the bike. They give you a big smile and wave you through magnanimously, while you struggle to get your feet back on the pedals and wave a less-than-heartfelt "thanks" to them.
It would be one thing if there was an unbroken line of cars zipping along at 900 mph and your only hope for making a turn alive is that someone, anyone, slows down a bit a and lets you go, but with normal traffic there are plenty of gaps and the person who does this always seems to be the LAST person in any line of cars.
It happens when you're trying to cross a busy intersection, too, or when you're approaching a four-way stop sign. The last 99 times things went a certain way, and you took your turn as if you were operating any other wheeled vehicle, but the hundredth time somebody changes the pattern, you're thrown a curve ball and you swing and miss.
On the list of nuisances that can befall a cyclist, this ranks pretty low. After all, when there are homicidal middle-managers in SUVs trying to smear you against parked cars, or drunken frat boys lobbing glass bottles at your head, someone being nice to you hardly counts as much of a problem.
But, aside from the fact that it messes up your flow and therefore totally blows your carefully cultivated sense of cool, there are a couple of problems inherent in this kind of behavior.
First off, it belies a common flaw in the way many Americans drive, which involves treating the brake and accelerator as on/off switches. You're either driving the speed limit or you're stopped, there's no sense of adjusting speed to accommodate road conditions. Not everyone drives this way, nor is it exclusive to U.S. drivers, but it's far less common in places where the streetscapes are more integrated, with pedestrians, cyclists and drivers adapting to each other on the fly.
The other reason this behavior is a negative indicator is that it means that, in some folks' eyes at least, bicycles are still a special case, not a normal part of traffic. On a bike you're still a bit childlike, to be treated with extra care rather than expected to be a responsible road user. This, in turn, reinforces the idea that bikes don't belong on the road with "grown up" vehicles, and makes it harder for cyclists to be accepted.
Overall, though, I'd rather someone throw off my groove by doing something unexpectedly nice, like waving me through a turn, than by doing something unexpectedly obnoxious, like accelerating to keep me from turning in front of them, or cutting me off to get to a parking space. Enough of the latter happens that when someone's unexpectedly generous, I'm more frustrated by my own uncool flailing than I ever could be at the driver!