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In this case it is because Amazon resisted the request from the legal system for the data.

The owner of the device giving consent to Amazon to release the data avoids having to play out the legal process of obtaining the data (it's likely the request would eventually have succeeded).

In this case, the defendant may have a say only because Amazon is claiming there is standing to enforce the defendant's First Amendment rights - the so-called chilling effect.

With the defendant consenting, Amazon's standing for litigating this issue becomes more fraught.

You could allow the police to do that without a warrant, if you wanted to. Amazon doesn't need to argue whether the prosecution have a legitimate order to seize the data if the 'owner' is willing to turn it over.