Definitely two broadly valid perspectives here(and a nice bow). There are plenty of instances where I wouldn't condone taking a tree without permission. For instance from a residential property, town public garden, or from a rare or historically noteworthy specimen.
I have seen enough fine timber get cut down and turned into garden mulch, firewood and landfill, or left to rot, crack and check until the timber is useless, that deserved a much better fate. Many land owners don't give a crap about trees in general, let alone understand the cultural, historical traditions and usage of timber.
How many yew trees grow straight enough and clean enough to provide a bow...not many. Surely it is a crime of sorts to waste such wood. On many trees cutting a branch can invigorate growth, the tree sends up new growth in clear, straight sections, providing subsequent generations with wood for bows. It called coppicing, and is a sustainable, renewable, and responsible usage of trees and timber management. The key to this system, is that it needs to be maintained. The slow growth of many trees means that in this day and age, the person who planted the tree probably won't gain any advantage from the tree he plants. Property changes hands many times over and the culture and its maintenance is lost.
What I said is a generalization. I'm not recommending anyone else to poach, or saying I would do it, but I certainly don't feel any less about someone who does choose to take a branch or even a tree providing it doesn't get wasted.
