Ultraman:

Bike shop labor rates are based on $60 per hour, and it does take about 20 minutes if the wheel is on the bike and the tire is on the wheel. So the $20 charge is right in line.

It is not true that replacing the single broken spoke gives you a wheel as good as new because of fatigue. Fatigue is stressing the material enough that it is weaker than it originally was. Breaking a single spoke could be a fluke. Breaking two spokes is dicey. Breaking three spokes is a sure sign of a wheel that cannot be trusted and should not get any more investment.

Jim Hughes:

Why does Trek sell substandard wheels? I'm sure there is variation from one wheel to the next. And you'd be amazed how many people under-stress their bikes. Trek probably has the data to show that the defect rate is acceptable. To lower their defect rate by, say 10% could cost them an extra 40%, so maybe they're at the sweet spot. This is where the bike shop can make a difference. I worked in one shop where, with every single new bike, we would remove the tires, true the wheels on the stand, and stress relieve them. Even the shop I work in now doesn't do that.