In order to connect this device (which was not a computer at all) to the Iowa State University mainframe across town, you would pick up the phone, dial the computer's number, listen for strange noises, and then slam the handset down into the rubber cups. If your aim was true, one would wrap its neoprene lips around the earpiece and the other around the mouthpiece, consummating a kind of informational soixante-neuf. The teletype would shudder as it was possessed by the spirit of the distant mainframe, and begin to hammer out cryptic messages.
THE CHARITABLE EXPLANATION is that the hardware monopoly policy reflected a drive on Apple's part to provide a seamless, unified blending of hardware, operating system, and software. There is something to this. It is hard enough to make an OS that works well on one specific piece of hardware, designed and tested by engineers who work down the hallway from you, in the same company. Making an OS to work on arbitrary pieces of hardware, cranked out by rabidly entrepeneurial clonemakers on the other side of the International Date Line, is very difficult, and accounts for much of the troubles people have using Windows.
THE FINANCIAL EXPLANATION is that Apple, unlike Microsoft, is and always has been a hardware company. It simply depends on revenue from selling hardware, and cannot exist without it.
THE NOT-SO-CHARITABLE EXPLANATION has to do with Apple's corporate culture, which is rooted in Bay Area Baby Boomdom.