Strepto is right... only the 2000 and 2001 RS got the rear LSD.
The way they market the AWD system (even for their cars without a rear LSD) avoids potential lawsuits from deceptive marketing tactics.
ie. If both front wheels are on ice while the rear ones are on pavement, the viscous center diff locks up and transfers most of the power to the rear wheels. Yet if half of the car (left side) is on ice and the other half is on pavement, without an LSD, the center diff never would lock up. All of the torque would be transfered to the tires spinning on ice since the open front and rear diffs naturally transfer torque to the tires that have no traction. To partially remedy this problem, Subaru equipped some of their models with a rear LSD to transfer at least some of the wasted torque to one of the wheels that has traction.
Yeah, it's not technically the best AWD system out there, but it is very light, very simple and still quite effective. It sure works well during the winter.