There is a certain amount of time that passes during which you are able to edit your posts (I believe it is set to 24 hours). After that time has expired, the posts become uneditable.
There are two reasons for doing this. The first - and main - reason is we have spammers who set up posts, then go back and add spam later, after the mods have been though. By limiting the amount of time they have, it gives us an opportunity to remove all spam. Secondly, from time to time, we will have two folks who just aren't getting along. It's not uncommon for someone to say nasty things, then go back and edit it before the mods can see.
Hope that helps!