How is it amazing when you had to wait multiple days for them to available? That seems completely backwards to me. Why aren't they available immediately once the block is confirmed, ie. within less than half an hour?
Fraud. If the transaction is too fast for new users, there's no time to identify and stop fraud before the money's left their control. Once you have the BC and can transfer it out, if you used stolen funds to pay for it, CoinBase loses twice the value transferred.
