I love these hypothetical groundless and utterly pointless debates.
The short answer is It isn't a problem, so don't try and stir it into one.
If BBs were do damn bad for animals, airsoft and paintball fields (because we ARE playing on designated paintball and airsoft fields, and not out in general wooded areas and in our backyards, right? RIGHT?) would be littered with the corpses of birds and squirrels and rodents and whatever other little furry forest animals happened to feast on our spent munitions.
That isn't happening.
It's not a problem.
If it still bothers you, use your afternoon volunteering at the SPCA or Greenpeace or WWF or something, instead of airsofting.