There are numerous websites that allow users/visitors to post content (such as comments to blog posts or news). The published post can be edited by the poster, but only for some amount of time (typically in the order of minutes). After that, the "Edit" button no longer appears and the post can't be edited.
My question is: what's the rationale for this behaviour? If it is technically possible to let the users editing their posts, why not offering this feature indefinitely?