a standard called the "rule against perpetuity." Any real claim that the BSA can have the land forever runs afoul of this premise. Any contract or agreement (and especially a will) that tries to violate this rule is typically found to be null. Basically a piece of land can only be encumbered for the span of "any person life time that is now in existence." (I don't recall the exact wording).
Had they put into the deed to the property as a covenant that it was to be only used for the boy scouts, it might fly, but even then, many covenants that were legal but discriminatory (like you cannot sell this land to blacks or jews) are ineffective, even though they were legal at the time.
It is possible that courts look at government grants differently...right of ways for rail roads, for example, seem to trump this rule, but I think that is in the deeds of the property that run with the land, not a contractual agreement.
I like the Boy Scouts and don't like seeing them dragged through the mud, but I do think they are on the wrong side of history on this, and they will pay the price...just as if they didn't want black scoutmasters. And I don't think they have a sufficient legal case to fight it.