Phillips' ruling in "don't ask, don't tell" follows a decision by Federal District Judge Joseph Tauro who separately ruled this year that the Defense of Marriage Act "plainly encroaches" on the right of the state to determine marriage." The administration has also appealed the decision in that case.
"They are not obligated to defend," said Belkin of the cases. "They already defended and failed."
Advocates say there is precedent, albeit limited, of administrations exercising discretion in declining to appeal the constitutionality of a law.
John Aravosis, author of AmericaBlog, noted one example from 1996 when President Clinton preemptively refused to defend in court a proposed law that would have banned HIV-positive soldiers from the military because he believed it unconstitutional. Aravosis argues Obama can and should now do the same.
"We were told that all hell would break loose if it ever happened," he wrote of Clinton's refusal to support the law. "All hell didn't break loose, a later Republican president didn't retaliate, and locusts didn't descend from on high."
When pressed to explain why the Justice Department has pursued an appeal of Judge Phillips' ruling, spokesperson Tracy Schmaler suggested the administration believes its obligation has not yet been fulfilled.
"The Administration does not support ['don't ask, don't tell'] as a matter of policy and strongly believes that Congress should repeal it," the Department writes in its brief. "The Department of Justice in this case has followed its longstanding practice of defending the constitutionality of federal statutes as long as reasonable arguments can be made in support of their constitutionality."
Schmaler said federal courts have been divided on the constitutionality of the law, making the question far from resolved.
Government lawyers have also argued that Phillips' worldwide injunction against the policy was too broad, necessitating an appeal for a narrower ruling to apply only to parties affected in the case.
"The 'duty to defend' is not in the Constitution, it's not in a statute, it's not in an executive order, but it is in the guidelines that govern the work of the president's legal counsel," said Beth Hillman, a constitutional law professor at the University of California Hastings College of Law and who also opposes "don't ask don't tell."
Hillman said the administration's actions could be reasonably viewed as an honest attempt to adhere to tradition. But she also said the administration may have taken this course to avoid setting a precedent that could imperil the new health care law, which is facing challenges on constitutional grounds.
"Let's say Obama is out of office before those legal challenges are resolved and then a president who follows Obama and opposes the health care reform bill and believes it not to be constitutional could fail to defend it. It's that argument that this would set a precedent in motion," she said.
Some legal scholars have also speculated that the administration may be reluctant to halt its defense to afford the Supreme Court the chance to issue an ultimate decision on the constitutionality of a law, potentially rendering it null for good.