Motors have been accepted in mountain biking since the dawn of mountain biking, that’s what an uplift vehicle is. So is a chair lift or gondola. You literally use a motorised device to turn some of the energy from its motor’s fuel tank or battery into potential energy stored in the bike and rider, which then provides the dominant power source for the bike as you ride downhill.
This same thing happened with skiing too, except skiing began cross-country, whilst MTBing actually started with downhill!
There’s also the strange attitude that it’s ok to drive a car to the start/finish point of your ride but not ok to ride an ebike - when my ebike rides are 99.9% from my house. It’s all very odd.