Our links to the outside world are as tightly restricted as our dialins. The Berkeley networking software is set up to support a much more integrated kind of network, where the network is treated as a single system. For our development environment that is much more effective. You should never allow that kind of access to a machine you don't control. The network software is weak.
Administrators are often unaware of the risks.
Not all hosts are trustworthy. The world is full of exciting challenges for attackers. All sorts of unrealistic simplifying assumptions are generally made. There are lots of equally devastating additional vulnerabilities waiting to be exercised, particularly in vanilla UNIX systems and networks thereof. Consider the following diagram.