Yes, Methane is poisonous, as a fact it hurts people, causing injury and harm. If you pay carefull attention to lawmakers, major producers of methane are sometimes questioned as being allowed to continue.
Though it is true that in an open area a composter will not pose a particular threat, if every home in the world has one it would be a problem.
This is why compost needs to be turned. Their is no other reason. Wether you know it or not you are turning it to disrupt the bacteria that produce methane as a biproduct.