They still won't do any harm to anyone else. (Or if they do, then either the traffic shaping or the billing is broken.) Still not a good reason to track where the traffic is going.

However, it would make sense to give everyone fully-automated metrics on their bill, letting them know their total transfer, along the same lines as what the upstream providers provide. Make it clear that it's perfectly acceptable to use what they pay for, and that by design there's no information about the type or destination of traffic, but inform them that if the usage looks unexpectedly high compared to their own known usage (provide equivalencies in "hours of HD video" or similar so they have a baseline), they may want to investigate for themselves.