Do you really want that to be the model for American justice? Law enforcement hands your digital electronic records over to a private company who has the power to put anything on that phone? Do you want to entrust your personal legal future to a private entity? I think I'd want a solid chain of custody on anything that may prove my innocence.

There's an article in the New York Times which says that Apple has already been doing this [1], but they seem to have had a change of heart in the past few years.

  Each data-extraction request was carefully vetted by Apple’s
  lawyers. Of those deemed legitimate, Apple in recent years
  required that law enforcement officials physically travel with
  the gadget to the company’s headquarters, where a trusted Apple
  engineer would work on the phones inside Faraday bags, which
  block wireless signals, during the process of data extraction.

1. http://www.nytimes.com/2016/02/19/technology/how-tim-cook-be...

Besides the chain of custody issues with the evidence if it leaves the Government's hands, Apple would almost certainly refuse to do this just the same.

Because once it is shown how it's done they will push to mandate the capability and force inferior security.