Leaving the IS turned on when tripod mounted can be useful in harsh conditions, However, when I mount the camera on a tripod I usually forget to switch the IS off. I would expect this lens to behave the way my other IS lenses behave and to sense the camera is static and not activate the IS.
And like I already mentioned, the "jump" and "drift" occur when composing for an upright shot.
I don't believe this is a normal behavor of IS mode.
Actually, I have used IS while tripod mounted hundreds of times since 1999. The trade winds here in Hawaii cause enough vibration to spoil a photo, so IS can be a saving grace if you use it with intelligence. Thus, it would be a bummer if the lens automatically disabled IS when a tripod is sensed! However under calm conditions lenses with standard IS, e.g., 24-105 IS or 28-135 IS, will often cause artifacts as they spasm, so you gotta turn it off. Luckily there is a switch on the lens, so it only takes a moment.
As for the "jump" and "drift," it was extreme with my old EOS 5, 10S and Elan. Oddly, it is far less apparent with newer bodies such as the Elan 7NE, 20D and 5D. Nevertheless, I notice it nearly everytime I use IS. I haven't owned a supertele with tripod enabled IS--are they smoother?--but my EF 300 4L IS USM, 75-300 IS USM, 28-135 IS USM and 24-105 IS 4L USM were very similar as far as amount of "jump" and "drift" with my newer film and digital bodies.
"I would expect this lens to behave the way my other IS lenses behave and to sense the camera is static and not activate the IS."
You must have dreamed this spec! It is not mentioned in the manual, the white paper, Canon USA website nor any reviews I have read. This feature is only used on the highend supertelephotos, e.g., EF 400 2.8L IS USM, as far as I know.