The reason that you're told not to do this is that forks in the first place aren't really meant to take any torque on them. The front fork holds your front wheel, but the rear axle is normally where the torque is applied when riding. Adding a system that then generates a considerable amount of force at that point can be acceptable for strong front forks, but most suspension forks may eventually give into the stress.

If you had an aluminum fork, i'd just suggest removing the hub, dont risk it. The danger of your front wheel coming off during riding is much more significant than a rear wheel oriented accident.

If steel, or another high strength setup:
My suggestion would be to check your fork regularily and make sure your lowers aren't becoming bent or weakened, understand the increased risks. Also understand that you may eventually permanently damage your front fork. Use some strong nylon capped bolts and a locking compound to make sure it doesn't come loose in there.

If you absolutely have to use a front drive kit, here is a suggestion:
Pedal first, or use a PAS instead of an immediate start throttle. The biggest threat to the integrity of your fork is right when you start. Not using the motor at that moment will make a world of difference.

The fork can definetly handle having the motor on there at cruising, its starting thats the issue.