The issue is not whether or not bird shot is always lethal.
In the eyes of the law there are no nonlethal applications of bird shot. If you shoot someone or some thing with a firearm as a civilian it will be considered the full force in the eyes of the law regardless of what type of ammunition you use.
Encouraging people to use bird shot by convincing them that it is not a deadly weapon is extremely irresponsible. It may be a sad excuse for a deadly weapon but in the eyes of the law it is a deadly weapon nonetheless.