If they could, they would read your mind too.
The Department of Justice is saying that a fingerprint is more like a key than a password I guess. Or alternatively that unlocking phones is search and not testimony.
The EFF seems to be primarily objecting on the grounds that a search of "all devices in the building" is too vague to be a correct warrant.
[1] http://www.uclalawreview.org/the-fifth-amendment-encryption-...
The self-incrimination clause of the fifth amendment is broadly interpreted as intended to dissuade the government from attempting to coerce confessions. Forcing someone to unlock their phone with a fingerprint caries none of the moral hazard of forcing them to testify.
All of that being said, I'd love for US society to recognize that a search encompassing a bunch of people's phones is vastly more intrusive (by virtue of the volume of data collected and network effects) than a search of a building that excludes computing devices, and for us to require a correspondingly higher burden of proof.
If you don't want anybody to be able to take your stuff, don't make it possible for somebody to take your stuff. A piece of paper saying something isn't permitted is not going to stop it happening.