You cant send them directly to exchanges just because your tokens needs to be sent first to a wallet that the investor control if it is not added from certain exchange. Since it comes directly from ICO, the possibility of it not being listed by the time of distribution is high and that's why it needs to be stored first on erc20 compliant wallet in order to add custom token and hold it there. If it was sent directly to exchange, then they won't credit it in user's balance.