Subaru has no choice. If you want to blame someone, blame the EPA. The stock 02 sensor (assuming it's the same as the 02-07 which it might not be) only measures down to 11-11.3 so, you're probably alot richer than that. On my 03, the fuel map calls for 9-10ish under high load/high boost.
How many wires go into the primary 02 sensor? If it's more than 4 then it's probably a real wideband and not the faux wideband all the older suby's use.