So you are saying the tree of good and evil was poisonous? If I had a poisonous tree at the cabin, I would chop it down so that the bear could not get at it.
Yes, 'poisonous' in a sense... that's a good analogy.
Then you would think that the course of action would be to chop it down, fence it off, what-have-you. But you have to realize that 'tree' is an analogy as well. The tree is what it represents. Tree of knowledge of good and bad = good and bad. Not confined to one area. It's something that just is.
Sort of like saying 'don't drink poison'. Not just don't drink the poison in the cupboard in your house (drano or something). But rather, don't drink poison, period. Because it is out there... it exists. You need to learn about it before you can touch it.
If you want to get into 'why' it exists, well that is an even deeper concept that I can only get a sense of. But the trees in the story, and the simple way that it is told, makes the concept of what they represent and what happened easier for us to grasp.
Peace,
tammy