While waiting for a good answer, here's a brief one:

For the technogeeks out there, SCSI lines are driven by open-collector (or open-drain) transistors. These transistors get their power through the termination resistors. So, proper operation requires both these resistors AND termination power. Each SCSI line pulls roughly 10mA from the termination power supply in the high state, and about 15mA when driving low.

There are so many SCSI lines that the power required to terminate all of them adds up to a value that is uncomfortably large for portable devices. So, the termination power needs to be supplied by an external source. Special low-power terminations (that do not fully conform to SCSI specifications) are used internally in Powerbooks (and the Portable). The Compact Macs have marginal power supplies to begin with, so even a single termination is pushing things a bit. The designers prudently decided to force external devices to supply termination power.