it has to be based on justice ?
But who said for instance in the bible that it haven't to be based on justice ? ... Justice for ourselve actually as a victime (like not putting any energy in hating someone and also alow you to be more objective about a situation = what is important and what is not ?) ... That's a way to lead to inner balance ...
It have a spiritual meaning ... You are talking as if the same action always gives you the same reaction (which is not true) it all depends on so many things/circumpstances (who/when/why/ ...) ... So it is not because you forgive someone that he/she automatically is going to do the same or bader ... Sur he can ... but nobody says that you have to stay in anybody's hand to be hurted ... and who knows he/she could grow up (at the end) but this doesn't really depends on the victime.
So as most people said here about forgiveness (to me) the biblical educativ sens of forgiveness is more about balance actually (in the most global sens) it doesn't mean you won't have to pay a bill (since everybody concerned have to deal with consequences) forgiveness in this process is not for nor against the offender but for victime (a way for him/her to keep the control of his/her energy = his/her balance).