They did it because all the other speakers were full range. They called the 6x9'* "subwoofer".

ROFL.

If I run the rear 6x9'* off of a separate amp, would it be safe to assume then that the factory amp will have the capability to push more power to the rest of the speakers given the absence of the 6x9'*?

Negative. The other speakers really don't need the amp. My amp has been totally bypassed. The after market radio is pushing all 8 speakers. At 17x4 RMS, they sound fine.

These things are paper cones, they don't take much to sound their best.

Except my front door 6.5'* and my rear 6x9'* are not factory original speakers. They're upgraded Sony Xplod speakers. The rears obviously I'm removing to replace them with some Kicker K693', and assuming they fit, I'll be putting a matching pair of Kicker 5.25'* in the rear doors.

My question was not whether they need an amp, but whether the Monsoon amp will have the capability of pushing more power to the other 4 speakers on the 4 doors now that the rears will be disconnected.

Oh, that I have no idea. I'm going to say no though, considering the amp was designed to push a certain setup. It'* not like a traditional amp that if you wire it different, it acts different.

Those Monsoon amps are require 2ohm speakers. They will not sound good on 4ohm or higher setups.

They do not require 2 OHM speakers, they just sound best with them. 4 OHM speakers will just be slightly quieter.

I though with the impedance mismatch, it would end up toasting the finals?

Nope not at all. Since there are very few good 2 OHM speakers, most people just use 4OHMs. It'* not that big of a mismatch to cause bad stuff.

Running speakers at a lower Ohm load increases the chance of failure, so if you listen to your music at very high volume, you would want to get the Ohm right. Having a higher Ohm load will give the speaker more control than at a lower Ohm load, which will slightly decrease power but increase the sound quality.