Why would the documentation say to wire a pole of the 3-way occupancy switch to ground?
Follow the manufacturer's instructions for wiring the device. If the instructions say make a ground connection, then make it.

For argument's sake, that ground could very well be for nothing more than grounding the switch (so it wouldn't necessarily be a pole).

However, it is likely that this device is indeed passing some current to ground. Below is a reference from a competing manufacturer of similar products. Notice that this only becomes a concern when combined with many such (twenty!) devices, or with other devices that are prone to leaking current to ground. Also note that this is only a concern on GFCI protected circuits.