Because the Scouts have never officially defined themselves as an anti-homosexual organization. This is really absurd. Until recently, it could be assumed that homosexuality was among the innumerable things most people implicitly disapprove of, and there was no need to spell it out. On the contrary, it was considered so base that it didn't have to be condemned overtly. Nobody supposed that if you delicately avoided condemning such vices by name you were committed to tolerating them.
Many moral traditions are, necessarily, tacit.
Justice Stevens didn't even refer to the Constitution and the reserved powers of the states. The wonder is that there are human beings depraved enough to inflict such cruelty on a helpless child. It's only slightly less wondrous that there are justices callous enough to bless it and confused enough to think it can be constitutionally protected. The Constitution has no more to do with the Court's opinions than the Book of Revelation has to do with the Unitarian Church. The Court's opinions are based solely on the Court's opinions.