I understand your point. However, a free-range flock is going to come into contact with the natural environment. Avian Microplasmosis is no joke. But with a free-ranging flock, contact is inevitable. In the opposite direction, even a chicken tractor would leave behind droppings that would contact a wild bird population. I suppose in an ideal world, there's a way to keep things cleanly and clearly delineated. For me (and I stress that this is only for me), I weighed the risk with the reward. I'm on a working farm on 20+ acres. My list of greater concerns starts with predation and rodent/vermin infestations. As odd as it may sound, my farm has fallen into a sort of rhythm with the rest of the environment here. I think it's important to look at your location and act within your comfort level. I'm aware of the risks, and I mitigate them as much as possible. Perhaps a better phrasing on my part would have been that, at this point, the wild bird population is not enough of a concern to warrant special action at my location.