It is only considered when a player is trying to claim free relief from a situation (IO, GUR etc) by taking a stance, swing or playing direction that is unreasonable.
eg a RH player's ball is 12" to the left of a distance post (IO) to the right of the fairway. He claims relief by saying wants to play backwards towards the tee. His stance for that shot would place him on the other side of the post and it would interfere with his swing. Such a direction of play would clearly unreasonable.
However, if a player has legitimately claimed and taken relief (from a path say), once he has determined the npr and legitimately dropped in the relief area, he may now play in any direction he wishes, whether others think it unreasonable or not. It is as if he was playing unencumbered from anywhere on the course.
The 'unreasonable' test only applies for the stroke to be made when he is claiming relief. NOT when he has taken relief.
In this case, if he correctly takes relief from the staked tree, his drop would take him clear of that tree but now leaves him with an unstained tree on his line of play. He is perfectly entitled to play away from that tree out towards the fairway.
However, the OP says he is allowed, not that he must. So even though he is entitled to take relief from the staked tree he doesn't have to. He may simply play it as it lies. In which case he may hit it towards the fairway.
