Can any one tell me why AEM recomends that cars with turbo should not get a by pass valve for their cold air intake?

 

the bypass valve opens up when the engine is struggling to get air into the intake manifold. (this usually happens when the filter is submerged under water, the bypass valve is to prevent hydrolocking the engine.)

In a car with turbo, the turbo is sucking the air from the filter harder than a NA motor would, thus the bypass valve would think that the filter is submerged under water.