The technicians for those tours are employed by the companies that go on tour with them. For instance, Clair Bros, Eighth Day, Sound Image, &c for sound; PRG, Epic Production Tech, Bandit Lites, Christie Lites, &c for lighting. These companies provide the equipment for the tours as well as staff the tours. Ain't an easy business to get in to, that's for sure.
I work for PRG out of their main concert touring hub/warehouse. Shoot me a PM if you have questions. We provide stuff for U 2's 360 tour, Lady Gaga, and Brittney Spears to name a few recent ones. Granted, I'm pretty new to it.