kun; unless it's clearly stipulated as part of the fee, in other words stated as gratuity added, you don't have to tip if you don't want to. But it would be nice for the crew.
Even in Europe, restaurants frequently add a gratuity that's not optional.
In this situation it is optional so be glad for that. So just do whatever you want. Nobody needs to justify things either way, it's neither right nor wrong, it's just the way it's done here.