The 4EAT has a fixed coupling to the front wheels and the variable MPT coupling to the rear wheels. The way the manual has the centre diff (including viscous coupling) at the rear of the gearbox which then connects to the front differential via a shaft that goes through the hollow gearbox lower shaft is an arrangement that I'd wager is not possible with the automatic, hence the autos use MPT.I'll be honest. I don't like the 4EAT AWD system. There has to be a better set-up. Is there any reason the 4EAT can't also get a viscous center diff? Why not use a front and rear Torsen diffs and a rear-bias planetary center diff. Throw some VDC in there and you'd have a superior AWD system, no?
The good news is that some have converted the 4EAT to use VTD rather than MPT, and VTD is probably superior again to the viscous LSD centre in the manual cars. See thread http://www.subaruforester.org/vbulletin/f89/mpt-vtd-conversion-37441/