The Declaration of Independence has been cited many times by the Supreme Court, including both sides of Dred Scott, Amistad, and a host of others. Having answered that objection, I should make clear my position that what the Supreme Court rules is less important than what the Constitution is and the expressed principles of just government are. Our government is comprised of three branches and many parts, up to and including the people themselves, not just the Supreme Court. The federal government shouldn't get involved as far as I'm concerned unless the states aren't adequately doing their jobs by protecting life as it should. It is clear that Oregon qualifies. My favorite part is, "The citizens of Oregon, through their democratic initiative process, have chosen to resolve the moral, legal and ethical debate on physician-assisted suicide for themselves by voting -- not once, but twice -- in favor of the Oregon act," as if moral truths can be decided by a democratic vote. Moral truths are moral truths, completely independent of tyranny of the majority. If moral truths could be decided by a referendum, surely the confederacy should have been allowed to continue with slavery.
As for what authority Ashcroft has, he has the authority and the duty to enforce federal laws such as the Controlled Substance Act and the Constitution. Whether or not the Controlled Substance Act is constitutional is subject to debate, but you'd be hard-pressed to argue that the Constitution isn't.
Second, I think think that you may find greater support than you think for the notion that "moral truths" have a role to play in government and law. Any disappointments that you may have might be attributable to our occasional inability to gain universal acceptance of what is or is not a moral truth. On such occasions, we need to find a mechanism to resolve our differences and democracy is just one of the possible mechanisms. But, to be sure, democracy is not for everyone.
Finally, the court in this case did not have to reach the question of the constitutionality of this attempted application of the Controlled Substances Act because the court found that the CSA did not, as Ashcroft was contending, have any application to Oregon's assisted suicide law. In other words, it suggested that if Ashcroft wanted to utitlize the CSA to interfere with Oregon's assisted suicide law, the CSA would first have to be amended by Congress.