Cult of Mac reports Cops can force you to use Touch ID, but not your passcode.
"A related feature has now been the subject of a court case in Virginia, however, with the judge ruling that cops can legally force suspects to manually unlock their iPhones using Touch ID. This differs from the current ruling related to passcodes. Police cannot force defendants to give these up on the basis that they are considered ‘knowledge’ rather than a physical object, and that knowledge is protected by the Fifth Amendment. A fingerprint, on the other hand, is considered to be more in line with a DNA sample or physical key, which means that citizens are compelled to give them up to police."
I hate when laws don't keep up with technology. This is similar to when you privacy rights were different based on if your ISP was a phone company or a cable company.