Yes, having been in both positions, I think it's a fair decision to let moderators ban members. I have trusted people to make the right decision and I would say I've also been trusted to make the right decision. It also depends on what your admin team looks like - I was once part of a forum where I was only 1 of 2 global moderators and we had 1 admin who never logged in over the weekend, so it was really down to us both to hold the fort. It was quite a busy forum too, especially when it first launched, so I wouldn't have been able to do my job without having that permission.