They explain how this works in their “tips” app - ie the user guide.
You seem to think that because you expect something to work a certain way, everyone does, and that’s just not true at all. For most of the history of iMessage, they were never synced. Eventually they rolled out the option to sync them with iMessage for iCloud. You can choose to use it or not. But I would suggest that just as many people think that deleting a text from one device won’t delete it from the others.
This is not the case of “apple” storing the message anywhere. This is the case of a user storing his messages locally on his Mac and then sharing the account with his wife. He’s clearly an idiot, but sure, blame Apple for not being able to save him from himself.
I’m not aware of the delete label in iMessage being labeled “delete from every device that you own and have signed into iMessage”.
There are numerous documented ways to avoid the situation he put himself in, he didn’t bother to find one and is now trying to blame others for his stupidity.