it's not a fixed or default fee, You can reduce/increase it but they recommend a certain amount to get a confirmation, I guess it's free to use any fees you want and it would depend on how busy the network is, Most miners are composed of thousands of users therefore miners would need to pay those to set a fee and even then people won't be forced to use it.
Actually you are right that fee is not fixed however it is always recommended to use enough fee so that your transaction get included by miners so if you only include like 10 to 20 satoshi per byte it may take more than 24 hour to get confirmed which will make quite panic situation to one who will receive those bitcoin as they can't spend unconfirmed transaction.
I always go with recommended fee even if fee is quite high than normal.