If you get a good valve, spring and retainer set, such as the trial or brash boy kits, or even the kit from monkeywrench racing, with all other internals stock, you can redline at 10K safely, the lift range from 6K to 9k can also be sustained longer, well beyond what the factory valvetrain can handle.
also remember this, sport bike engines have way less displacement, and much lighter internals, they can easily rev much harder than a car engine; ther's less stress on the valvetrain, and it's designed a lot differently.
Rotary engines can also rev high (some get wankel rotaries to rev as high as 12K!!!) this is because there are less internals to break, and they operate on a totally different principle than a piston-based engine.
But believe this, the 2zz was made to be revved hard, with a valve upgrade, you can realize it's true potential.
BTW, one more thing, some race-built spoon motors can achieve 15K, but not sustained, the sustained rev limit is about 10-12K. anything beyond that is for limited use (like hitting lift hard in a stock celica).