A flag is only that, a piece of cloth that only exists for the symbolic value superimposed on it. A flag serves no other purpose, in principle. (Well, you could perhaps try to use it as a jacket or a shopping bag but that's not what it's intended for). The tree is a living biological entity with its own independent existence, with a symbolic value superimposed on it. If you have to destroy it because it has become "soiled" in that symbolical sense, but only in that symbolic sense (biologically there's nothing wrong), it becomes more like a sacrifice, in my eyes, (an animal that had to be slaughtered because something in the symbolic, or at least immaterial domain went wrong, for example to "appease" a god). Besides, even if the flag has been soiled (but not damaged beyond repair), there is no objection to simply cleaning it according to that code, as I understand it, at least. Burning is only called for when the flag is so tattered that it cannot be used again. If they can think of that provision for a simple cloth, why can't it be done for a living thing? (And that's not even considering my own opinion, which would be that after the piece of cloth served its purpose as a flag and is now worn out, it simply loses to have that symbolic meaning and can be disposed of as seen fit; it's the symbol that is important, not the material object. Having to burn it in an "honourable" way would feel to me as symbolism taken a bit too far, but I'll agree that's personal.).