It's not necessarily that they're restrictive. They're designed to meet a target hp number and to last as long as possible, all while passing federal emissions standards placed on new cars (these are not the same standards you have to pass on a state government's tailpipe sniffer). Aside from the conservative calibrations I have already mentioned, turbo systems are designed to have headroom to flow more air when it's really hot out or the car is at high altitude so more boost is needed to make the same power level. If you crank up the boost past a certain point the turbo is no longer able to make up the difference beyond the extra horsepower it's already making.

Don't you think that if VW or Subaru could make their motors last just as long and run just as clean while running higher boost levels, they would? Every factory turbocharged engine is pretty conservative in its tuning but so is pretty much any factory engine. It just so happens that it's easy to exceed the factory settings by a good margin with a turbocharged engine.