But this fee you say they charge, from my understanding this fee goes to the miners and not them(coinbase) unless they don't apply the said fees to a transaction then thats dishonesty on their part!
Of course Coinbase spends less on the fee than they charge their users as for them that's just another revenue stream and it's not really dishonest as they are not hiding their withdrawal fee.
Truth to be told, Coinbase is not making much compared to Binance who has much higher withdrawal fees (at least for bitcoin).