Mark Kleiman and Eugene Volokh are apparently both of the opinion that the Boy Scouts' policy of excluding atheists amounts to a kind of religious discrimination. (Both concede the Scouts' Constitutional right to their policy, but consider it morally wrong nonetheless.) As Volokh puts it, "[i]f the Scouts excluded Catholics -- everyone else, Jewish, Protestant, or what have you is fine, but not Catholics -- we'd rightly condemn them, even if they said 'Rejection of Catholicism is one of our core beliefs.' Likewise, I think, when they exclude atheists." (Kleiman makes the same point, right down to the choice of analogy.)
I rather doubt that the Scouts' opposition to atheism is as narrowly defined in practice as Kleiman and Volokh claim; would the Scouts accept, for example, a Satanist troop, or one that worships only a particular (living) charismatic cult leader? If the Scouts turn out to have bona fide doctrinal standards compatible with most religions but exclusive of a few, then they would be no different from a group that rejected, say, believers in performing child sacrifice rituals or murdering all heretics (except, of course, in the sense that Kleiman and Volokh probably find adherents of such ideas far more worthy of exclusion than those, including atheists, who happen to offend the Scouts' somewhat woolier religious principles).
I also wonder whether the two professors have paused to consider in just what company they have placed themselves with their choice of analogy. The most conspicuous advocates of the idea that atheism is a religious conviction--comparable to, say, Catholicism--are fundamentalist Christians attempting to inject "Creationism" into the public-school curriculum. After all, if atheism is a religion, just like literalist Christianity, then it's perfectly valid to claim that Darwin's theory of evolution is as much a religious position as is the "Genesis theory" of human origins. Likewise, if absence of religion is just another religious creed, then school voucher programs that encompass confessional schools are not only Constitutionally permissible--they might even be mandatory, under the judiciary's current broad reading of the First Amendment, to prevent the government from "establishing" atheism, over all other religious doctrines, as the official "faith" of the public school system.
But atheism is not a religion; the absence of religion is very different from the presence of one. Teaching evolutionary biology in science class, and rejecting all of its religious alternatives, is not the same as teaching a single religious alternative. Forbidding the promulgation of any religion in public schools is nothing at all like exclusively promulgating a single one. And likewise the Boy Scouts, in requiring their members to affiliate with a religion--any religion--are not "excluding" a particular religion. Kleiman and Volokh should be happy about that; the consequences of atheism being designated by convention as just another religious belief would be most unlikely to please either of them.