I suspect that if you think about it logically, Servo mode is to track a moving object, once it has acquired that object it will continue to track it, moving the camera to a totally different object that is nowhere near where the lens had it's last registered position is no doubt got the lens saying hey where did it go!!
AIServo is for tracking objects not selecting a new target.
Not strictly true.
AI Servo is meant to continuously focus on whatever is under the selected focus point, or rather, to ensure that whatever is under the selected focus is point is always in focus
Nighthawk is correct. And this is the way that all lenses work. Try it. Put the kit lens, or a prime, or whatever, on your camera, and AI servo will focus on the closest object that you point it at, and as you move the camera around, the focus will shift to the closest object that you move it to (aside from the temporary effects of fine-tuning the AF system with the cases). The camera has no concept of "objects." AI servo acts like it is following objects because it focuses continually, and you are expected to keep the object being photographed under the active AF points. As the object moves away from the original focus point, the camera looks for something in focus at the same distance (i.e., under an adjacent active point), and shifts the focus point accordingly. However, if you don't keep the object under an active point, the lens will focus on the closest object under the active points (which may be miles away!). Unless it doesn't, as with some 150-600 lenses, and in those cases the lens is not operating properly. 
