Ironically, the reverse is true. Real memory foam is very dense and does not let air through. That means a real memory foam topper is warm to sleep on, because there is no ventilation to remove the warmth produced by your body. Fake memory foam, on the other hand, is usually made from cheap foam, which is porous. As such, there is natural ventilation to cool you down.
So why buy a real memory foam mattress topper instead of the fake? You buy the real thing when you want to sleep more comfortably and for its longer lifespan. Besides, why buy the fake when you can get normal polyurethane foam mattress pads much cheaper than fake memory foam pads? After all, they are the same thing.