They are also banned by many home ISPs so even though this is already an impractical solution to host a forum or perhaps even a BBS (if you want to go retro), there are also legal implications to this as well.
Really? I know ISPs have been forced to deny access to for instance TPB, but I haven't read any stories of actually blocking the torrent protocol. That would be a huge violation of net neutrality, and it's actually forbidden for ISPs restrict torrent bandwidth.
That being said: decentralization comes with another problem: you don't want every forum users to see IP addresses of every other forum user. So a VPN or even better Tor would be required anyway, and the ISP won't know you're using a decentralized forum with magic internet money.