Noisy brakes --WTF?

Driving with 2 feet (one on gas pedal, other on brake) instead of using

2 foot for both pedals (to ensure you're either gassing or braking but not both at the same time). Another cause of brake drag could be a partially engaged parking brake, or a poorly calibrated one.

Did they inspect the calipers to ensure the slider pins were clean and lubricated?

If both rear brakes where bad, I'd suspect a bad proportioning valve. Not very likely. More likely just one side was bad. Normal practice to replace both sides when only one side has a problem. Likely one of the pads had a manufacture defect: too soft or too hard. Your driving style not the cause here because the front brakes do the majority of the stopping. Fronts would have worn out first if the way you drive was a factor.

Not E-brake since that uses the 'hat' and it's own 'shoes'.

Has to be some kinda manufacturing/parts problem. I've heard of hydraulic hoses that have 'flaps' inside that act like check valves - or just some pad installation or defect as others have said. I also wonder if the ABS could be an issue?

What is this car using for an emergency brake? My old car uses the front brakes and that could cause your problem if they are using the backs now.

There is a seperate small set of shoes that operate the parking brake on 'most' subarus. You can seea little of that detail about halfway down here;

Carl