Why would they? They charge a flat fee for any amount of Satoshi being withdrawn from thier exchange. In most cases they make a tone as from just withdrawal fees. Another way to earn Bitcoin without doing nothing.
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!
All in all to avoid transactions being stuck in limbo and ensure its confirmed in the next block a high fee is applied or in layman's terms charged..