Coming from Alberta, I know it's not a problem unique to first nations. When operators get trained, whether they're with a first nation or another small community, they're a very valuable commodity and often get recruited away.
I recall testimony that we heard from the Piikani first nation in southern Alberta. Their manager said that it's a glass half-full, glass half-empty thing. He actually found this to be an opportunity for people on his reserve. If he trained them up and they became skilled and could find employment elsewhere, then he wasn't about to hold them back.