From what I've read, they cannot offer a turbo version because they signed a contract with Toyota saying they would not modify the basic engine layout for 2 years (obviously very few that could afford it would want a Scion model with no power if you could get a Subaru Turbo one with 350HP at the crank for only $5-7k more (i.e. Base WRX versus old basic Impreza). Thus, they're going to have to do things like lighten the car, program the engine to let it rev a bit higher, etc. Carbon fiber isn't cheap and it's not an effective replacement for a turbo, but they have no choice due to this contract they signed. It's the only thing they're allowed to do, but of course they'll spin it to sound like it's a good idea and they "want" to keep the turbo lines separate for the WRX STi, etc. That's a load of horse manure in reality, though. They put turbos in a lot more than the WRX STi. It's not only in the regular WRX, but it's available in the Legacy, the Forester and formerly the Baja as well. Subaru doesn't want to admit, but most of the dealers will tell you the contract they signed is the reason there's not going to be a turbo version (STi or otherwise) any time soon.
It doesn't make good business sense at all to leave it N/A. This car could compete with the Porsche Boxster head-on with a turbo in it (in both looks, cornering and performance). The WRX is AWD and 4-door. There IS a lot of overlap with some customers, but for others it's a different market segment altogether. Some want AWD (launch, handling and winter capability) and no RWD sports car is going to change that. I know 2-door coupes typically look a bit nicer, but 4-door functionality is pretty darn nice for actual usability unless you almost never drive with more than two people in the car. It's not really a sports car, though. It's a high powered sporty sedan. Sports cars are something you use in nice weather as a secondary car. Only poor people use them as their regular transportation and they're probably giving up something else...like solid food to get a true high powered one, not "sporty handling" cars like the Miata (or now the BRZ) that lose their lunch on the straight-ways to many V8 pickup trucks and even some high-end Mini-Vans. If great handling is all you care about, fine. I maintain my ProbeGT (7.4 seconds, so similar to the BRZ) was a lot of fun on twisty country roads. It went around corners great (contrary to the beliefs of some that hate American cars; it was half Mazda after all), but my daily commute was a lot of straight roads and freeways and you couldn't help but notice large block engine vehicles (even trucks, etc.) that seemed to effortlessly pass you on a normal launch from a light while you had to rev it to the line to keep up or pass them. I can guarantee you I don't EVER have that problem with my WRX. I rarely floor it and I still leave 95% of all vehicles in the dust without even trying. To me, that's worth the difference in price.
No, it's just the contract and you can see why either side would agree to it. It's about risk management and recouping the investment costs. If one outsold the other 10:1, someone would walk away a clear loser. This way, it's just a badge and maybe little things like the exhaust and some weight reductions, etc.