Why DisplayPort cables must isolate the power pin?
You are shorting two different Voltage Sources together. Both will compete to provide the power on the line. Not a good idea.
From http://www.displayport.org/faq/

Q: Why isn’t there a wire on pin 20 (POWER) on the standard external DisplayPort cables?
A: The reason power isn’t included in standard cables is because both source and sink devices are designed to provide power.  Captive, attached cables often include the power wire. If it is desired, for example, that a particular source device utilize the power available from the mating sink device, then that Source device could include an attached or dedicated cable that carries the DisplayPort power signal. Same could be applied to a sink device.

Since Displayports allow for daisy chaining and other features, sometimes its good for a sink device (like the monitor) to provide power. Sometimes it is not. A direct connection between a Source (like your Computer) and the Sink device is not a situation where both power pins should be connected/used.