I would guess that the FBI never asks Microsoft for encryption keys without a valid legal order because it knows Microsoft will demand one, and because the FBI rarely has possession of suspect devices without a warrant to search for them and obtain their contents.

It could be a bigger obstacle for other agencies. CBP can hold a device carried by someone crossing the border without judicial oversight. ICE is in the midst of a hiring surge and from what I've read lately, has an abbreviated screening and training process likely not matching the rigor of the FBI. Local law enforcement agencies vary greatly.