They're not loaded from the server but the sms.db (sqlite) database on your phone.
Tools exist to read the database from the phone directly, from backups, etc. Which is great when you accidentally delete them... I wouldn't know that though. :)
That's not what's happening with the "load earlier messages" button at all. The long term iMessages history is stored in an unencrypted sqlite database on the phone (which is included, unencrypted, in local iTunes backups).
Local backups can be encrypted (mine are). Also it's not necessary to use cloud backups (I don't).
Therefore, my iMessage history is secure (I fully understand that my message recipients may expose my individual messages, but my copy is safe).
The fact that it is possible to perform encryption on the entire backup file does not change the fact that many people are (wrongly) speculating about things which are easily verifiable.
Re:Can Apple read your iMessages?
by fortepianissimo on Wednesday June 26, 2013 @04:39PM
(#5948860)
But what's the difference? Whether the messages are stored in plaintext, or whether they are stored as part of the iCloud backup, the author proved that Apple can get to them in order to restore your newly purchased phone.
Also.. it's important to note that the iMessage client only retains about a day's worth of messages on the device. Earlier messages can be obtained via a "load earlier messages" button. This means that Apple does store the message history on the iMessage servers, and there is a way for the client to retrieve them in batches while the contents are not accessible to Apple.
It seems extremely likely that this 'iCloud Backup' vulnerability is a red herring. The messages are not stored in the regular backup but are fetched encrypted by the client when you sign in.