Technically if it's true that THC is used to basically give the small animals and inscects an effect before it can damage the seedpod, giving it time to ripen then it is a poison.

A substance, which upon contact or being introduced into an organism, impairs or prevents normal metabolic processes from taking place, thus altering the normal functioning of organs or tissues.
Now it might be considered a poison for animals but we as humans are probably ten times the size of the creatures that disturbs the plant, plus we have enough sense not to fuck with a flowering cannabis plant, we'd be wanting to smoke it so we know when to mess with it . so instead of making us hella confused like the creatures it's probably just pleasurable to us, until we get too much in us of course, then I would consider it a poison haha.