I don't post often here, but I do stop by frequently, so I can totally see where the band is coming from. I enjoy reading useful and constructive posts, but it takes me forever to find those (there are many perfectly sane people on here who don't post frequently).
I participate actively on two other boards, and I casually frequent a couple of others, and I just have no idea what it is about Opeth that attracts this type of "following" and "immaturity". Sure, I too was once a 15-year old, but I never talked to people the way that some talk to others here. Some posts are really insulting, bordering (at best, I am being kind) on racism, ethnocentrism and insulting people with disabilities.
Sure, I'll miss stopping by here myself, but really, I still have my Opeth music and I will go to many Opeth shows in the future. So, shutting this down is not a big deal, and it is a good move by the band to distance themselves from this type of behavior.