From asking on other Forum sites, the answer to those questions that I got were that firstly, if every poster can edit and delete posts at any time, then threads end up not making sense; and secondly, Mods and Admin have a load of cleaning off spam and escort services and porn posters and the like, and then have to send warnings to members over posts that break the rules, so it just becomes impractical to also email every poster in a deleted thread.
Makes sense, considering that the sites are free and most Mods/Admin bods are volunteers..... I do know what you mean though, about it being a bit disconcerting when something or someone's just gone.