What Martok said.
We plainly list in our TOS that the accounts are part of the property of the forum.
There are many reasons for this, including those Martok listed.
People get angry, want to delete, then they want back.
Screws up the discussion threads and info.
Screws up member numbers (think advertisers who want eyeballs)
We have a group for "former members"- they get put into that group and their banner is "Former Member" so people know they are gone.