In defense of the ISPConfig's settings, I have worked at companies where deleting of spam is a cardinal sin. One man spam was another's million dollar deal. What we did was store all spam for later retrieval.
I have not tried out the spam bucket
It would be interesting to integrate something like: Maia Mailguard. Something to give a per user instead of per site manager access to spam settings.
On my system, I don't want to deal with that and delete incoming spam.
It is what I would call in my day job a "Management decision."