Which means they are not really encrypted. Sure they run the algorithm, but I am not in control of the key, so who cares.
A locked door is still a locked door, and serves to prevent everyone who doesn't hold a key from entry.
For iCloud, the user has a key, and Apple has one, which it can be compelled to use under warrant from law enforcement.
By your reasoning, I suppose everyone who lives in an apartment shouldn't bother to lock their doors at all, since the landlord also has a key (where they can be bound by law not to utilize unless they have given prior notice). I think it's safe to say tenants do, and should care.
You may argue that in these situations, that security is not absolute, but that doesn't change the fact that there is a layer of security present. Those who desire greater security would of course be advised to keep their backups local.