Now suppose that you have invented a great new way to make computer networks, and that it is significantly more convenient than any other way of making computer networks. In fact, these networks are so fast and so convenient that everybody is buying them. Dammit, it doesn't work that way. The world is full of people who care only about expediency, about getting their screws driven or their nets worked. In the heat of the moment, they are not going to remember the caveats.
People never do.
If the only computers were on military bases, you could forbid the practice and punish the offenders. Some guy in the Petroleum Engineering department buys a computer, gets a BSD license for it, and hires a Computer Science major to do some systems programming for him. The CS major hasn't taken the networks course yet and doesn't know the risks of breakins. The petroleum engineer doesn't know a network from a rubber chicken, and in desperation tells the CS student that he can do whatever he wants as long as the plots are done by Friday afternoon. Then he needs to copy it to the CS department computer, so he puts a permission file in his account on the CSD computer that will let him copy his homework from the petroleum engineering computer to the CSD computer.