The majority of requests are made because someone suspected of a crime (pick any crime: fraud, possession of child pornography, drug dealing) - and they believe there to be evidence on the device.
If we prevented legal authorities from accessing data on your phone, then in theory a peadophile would only have to ensure he/she downloads illegal images to their phone to ensure they cannot be caught.
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The reason we have warrants is to ensure there is a second layer to verify that a search is proportionate. Obtaining information from a phone should certainly require a warrant issued by a judge; but it certainly should not be completely unobtainable.