My two cents, if I may, as ignorant Devil's Advocate.
Considering what has been suggested - that we are raising a nation of sissies - isn't it prudent then, simply because of the way things are 'now,' that these kids have the perceived safety of an adult present? Times have changed and some of it's unfortunate. This new rule about over nighters is in response to the way kids--and some adults, no doubt--are now-a-days. We might wish the BSA had tried to hold on to some of those traditional values that encourage responsibility, accountability, independence, etc--the same values that these kids may be missing--but it is probably hard pressed to, considering, as suggested, the way kids are now being raised. It's a legitimate reaction to our changing society, especially considering the lack of personal accountability that is running rampant in the mainstream (stereotype-pushing example: "We're gonna sue 'em! My boy twisted his ankle because no adult was supervising!").
I'd guess that this change was motivated from Lawyers/Insurance providers?