Copying documents across collections with modification to _id doesn't work

When a document is retrieved from a query, the _id value modified, and then inserted into a new collection, the _id value is ignored and the document is assigned a new ObjectId.

> document
{
"_action" : "",
"_id" : 189993,
"_process" : "",
"_status" : "",
"author" : "devadmin",
"category" : "Uncategorized",
"content" : "when a new post is added to the Hero Module on GameFront's Homepage, the post is hidden from the RECENT ARTICLES module. Please remove this logic so posts that are added to the hero module can also appear in RECENT ARTICLES.",
"published" : "2012-12-24T09:17:40Z",
"source" : "gamefront",
"summary" : "",
"tags" : "Reviews",
"title" : "A new post that test when post is in hero module",
"type" : "post",
"updated" : "2012-12-24T09:38:01Z",
"uri" : "http://www.gamefront.com/a-new-post-that-test-when-post-is-in-hero-module/",
"thumbnail" : ""
}
> db.gamefront.insert(document);
> db.gamefront.findOne();
{
"_id" : ObjectId("517ac61539039707d1f6a250"),
"_action" : "",
"_process" : "",
"_status" : "",
"author" : "devadmin",
"category" : "Uncategorized",
"content" : "when a new post is added to the Hero Module on GameFront's Homepage, the post is hidden from the RECENT ARTICLES module. Please remove this logic so posts that are added to the hero module can also appear in RECENT ARTICLES.",
"published" : "2012-12-24T09:17:40Z",
"source" : "gamefront",
"summary" : "",
"tags" : "Reviews",
"title" : "A new post that test when post is in hero module",
"type" : "post",
"updated" : "2012-12-24T09:38:01Z",
"uri" : "http://www.gamefront.com/a-new-post-that-test-when-post-is-in-hero-module/",
"thumbnail" : ""
}