The assumption is that some (crazy) people are running tor exit nodes on IPs shared with other traffic. That is IMO a seriously bad practice; even putting stuff like that on the same /24 as critical servers is probably a bad idea.
Part of the goal of Tor is to have a bunch of normal people running Tor nodes on their own personal machines, VPSs, etc., in which case they're almost always shared with other traffic (the person's own traffic).
The topic of this thread is that if you're running a Tor exit with a limited exit policy you'll get banned by lazy admins who presume that all Tor exit nodes have a promiscuous exit policy.
There's theoretically nothing crazy or bad about running a Tor exit node like that, e.g. one that only allows exits to a few sites that you yourself control.
But in practice it's bad for you because of people that think "let's just nuke it from orbit" is responsible behavior on the Internet, even going so far as to ban whole /24 networks.
