Even if that's the case Tim, I'd assume you're usually panning with your camera on the tripod, or at least have both hands on the camera, in which case, even on the tripod, the IS system will detect enough movement through your hands to operate just fine. Using the camera on the tripod with the head completely locked off, where no movement at all is detected, would cause problems with earlier IS versions.
My 100-400L is around 3 years old and I use it a lot on the tripod -usually with a gimbal head - with no problems at all.