You might want to check your history a bit better, Picko. Nintendo had policies that said that developers who jumped ship to competitors could not make games on Nintendo systems; they did not ban these developers from putting games on other systems entirely. If you mean that in effect they banned it because nobody wanted to jump ship from Nintendo, then perhaps you missed the underlying fact that Nintendo was the only hardware manufacturer in town that mattered.
Developers chose not to move over to the Sega Master System out of fear of failure. Success was assured on the NES, but not on the SMS. Nintendo didn't have to lift a finger; a simple "you leave us you can't come back" policy was sufficient. As with a proper market, things sorted themselves out naturally and a perfectly reasonable policy (one which, I might add, developers such as Coleco and Atari tried to enforce as well less than a decade prior) only ensured that competition was minimal.
If you plan to criticize Nintendo for minimizing competition in the 1980s with exclusionary policy, then I suggest you also throw some of that criticism at Microsoft for buying support away from competitors, and at Sony for buying the market at large by offering everything dirt-cheap and taking a loss. These are no more or less egregious offenses than Nintendo's passive policies concerning third-party developers.
Sky Render - Sanity is for the weak.
