When I think about it, I don't understand why Apple wouldn't want to help this effort and just provide all the documentation. All the classic reasons ("competitive advantage", "secrets", etc) do not hold water in this day and age.
The real answer is probably simpler than anyone here is making it. Apple hardware margins are healthy enough that selling macbooks to linux users is pure profit, so no services lock-in needed. However, the moment they officially acknowledge Linux support, then it becomes a support surface. Every kernel panic becomes a genius bar visit. Every driver bug becomes a tweet at @AppleSupport. It's the value of plausible den…
What do you mean by needed? A lock-in is more profitable so is needed to maximise profits.