For example if you have 3 devices (iPhone, iPad, MBP) and someone goes to send you a message, they have to re-encrypt the message three times because Apple would have sent them three public keys.
Now if Apple were evil because of a government order, they could send down four public keys, the three ones for the devices you own, and the one public key that Apple has the private key for. At that point once they receive the message they can read it.
Any system that distributes public keys like this can be compromised the same way.
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The only real way to stop something like this is to make sure that the person you are talking to holds the keys, OTR does this for example by allowing both parties to verify the fingerprint...