I've never been close enough to anything produced by MD to verify this, but I believe the NOTAR system includes a duct along the tail boom, which provides (via the Coanda effect - thanks Phil!) anti-torque force. Presumably, when transporting by road, especially in the wonderfully wet excuse-for-a-summer we're having, you don't want water and things blowing in via the duct, so you stick some bin-bag over the hole. As someone's helpfully painted the reg over the duct as well, this has some side-effects.
Just guessing here?