When it does, there are usually somewhat weighty reasons for it. As I think there are here, albeit regarding an issue upon which reasonable minds could surely disagree. Here are the concerns that motivate Judge Nelson to concur:
"The majority recognizes the possibility that an interpretation of “terrorist activity” that ignores international law could lead to some bizarre outcomes, including classifying as terrorists Jews engaged in armed resistance against the Nazis. Maj. op. at 12774. But such anomalous results are not merely hypothetical: the United States military, whose invasions of Afghanistan and Iraq were indisputably “unlawful” under the domestic laws of those countries, would qualify as a Tier III terrorist organization. Accordingly, any individual or group who assisted the U.S. military in those efforts would be ineligible for asylum or withholding of removal. [Cite]. This could discourage sympathetic groups from lending support to the U.S. military, knowing it would preclude them from seeking refuge in the U.S. in the future.
The majority contends that such concerns are overblown, pointing to a provision in the statute allowing the Secretaries of State and Homeland Security, in consultation with each other and the Attorney General, to waive the terrorism bar. [Cites] I hope my colleagues are correct. I, however, am less sanguine than they are about the efficacy of this waiver provision. First, the waiver is entirely discretionary and unreviewable. [Cite]. Second, the waiver requires the assent of three separate agencies, posing a daunting bureaucratic obstacle to implementation. Third, even without this high administrative hurdle, a waiver seems to me a haphazard and inefficient means of avoiding outcomes—such as classifying the U.S. military as a terrorist organization—that Congress clearly never intended. Finally, because India is a democracy, the waiver provision is not even available in this case. [Cite]"
Whether such an unlikely (and I think it is indeed unlikely) scenario is a sufficient reason to go another way on a legal principle -- albeit an important one -- is a matter of judgment, of course. So I think both sides to this debate make some sense. As well as darn good points.
Just proving yet again why it's important to have smart and interested people on the bench.