It's because cryptocurrency withdrawals are free on GDAX (
transaction fee's are always showed before withdrawal). See it here:
Deposits and Withdrawals
Crypto Deposit: Free
Crypto Withdrawal: Free
I really have a feeling that this will not last long because aside from the other cryptocoins shouldering bitcoin fee's for a long time is just unsustainable.
I'm guessing this is just another strategy to gain customers?? nevertheless it's still a good thing so we should use it until it runs out