I don't pretend to be tapped into the hivemind of the Nine, but I don't think that's the case. Both sides dress their arguments up with issues of authority, but the underlying theme seems to be whether or not assisted suicide is a legitimate medical purpose.
Consider Scalia's dissent. First, he argues that Auer gives the Attorney General the authority to make the decision as he sees fit. But then he turns around and offers this:Translation: "Even if you think the Attorney General doesn't have the appropriate authority, the statute clearly says that doctors can only prescribe drugs for a legitimate medical purpose, and there's no way in heck assisted suicide qualifies."