I would see that different. A dust boot that moves with the Z-axis seems wrong in many ways.
if it would move with the Z-axis (up and down) then it would change the contact as you cut. this would be problematic as the deeper you cut the closer to the material you would place the dust boot, at some point you would have interference and or contact with the material or worse yet the clamps etc.
with a stationary Z but mobile with the X-Y axis the distance from the clamps and material would be constant and the dust boot follows the cutter as it travels along the path. this makes more sense as it is less likely to interfere with the cut. I have enough problems getting everything correct without another obstacle to worry about like the dust boot catching on a clamp or anything else in the path.
Just my point of view but I would like to know why it would be better to have the dust boot have Z-axis movement up and down as you cut.