Yes, this is probably a stupid practice. But if you're stopped at a red and see a biker do this safely, without causing disruption, and that pisses you off, then that is 100% your problem not the biker's.
Actually it's most likely not legal. (at larger intersections, of course this is not the case everywhere) In most countries (all?) it's illegal to do a u-turn when there's a double line between lanes and usually they're painted a fair distance before intersections to prevent people from overtaking when they're close to lights / stop signs / intersecions which they may not see yet. On a bike you're really unlikely to get to a legal distance from an intersection and come back in time for the same green light.
Also it's confusing pedestrians crossing the street - you expect vehicles coming from one side or the other, not suddenly appearing on your left, because they did a u-turn right after they passed the crossing.
That's good and I agree (and sorry if that came off too antagonistic).
It occurred to me that what I was responding to was actually the notion (which always comes up in this duscussion) that an appropriate response to a discussion of "How not to kill cyclists" is to say "well, but they shouldn't do this thing that annoys but really has no effect on me".
Except it does. An intersection is one of the worst places to be "pulling a maneuver." There are too many things happening from too many directions, and vehicle and pedestrian traffic are intermixing.
The base reason we have traffic laws is for safety. A good part of traffic law is flow: direction and speed. Anything with a wheel is a vehicle, and all the vehicles are using the same space. When you have all that lethal metal (and bikes are also lethal metal) moving around, it all needs to be going in approximately the same direction and following approximately the same rules.
At an intersection, you have all these things ... intersecting. And now you throw a bike into the intersection, moving across expected flow. It's unnecessary risk for everyone at the scene.
It only has no effect on me if everyone is lucky. But it still adds risk, to me and everyone else.
