Hmm.
http://www.faqs.org/faqs/scouting/rec.scouting.issues/section-11.html (“Q. What was Baden-Powell’s position on God and Religion in Scouting?”)
Though there’s no mention of Christianity or religion in their Wikipedia entry. There is this though,
“Duty to God” is a principle of Scouting, though it is applied differently in various countries.[46][47] The Boy Scouts of America (BSA) take a strong position, excluding atheists.[48] The Scout Association in the United Kingdom permits variations to its Promise, in order to accommodate different religious obligations.[49]
Is this where we’re getting “Christian organisation” from, the US implementation?
More interestingly, and in answer to the OP, it goes on to say,
in 2014, United Kingdom Scouts were given the choice of being able to make a variation of the Promise that replaced “duty to God” with “uphold our Scout values”
The Scout Promise has its own entry,
https://en.wikipedia.org/wiki/Scout_Promise#United_Kingdom
Variations to the wording are allowed in order to accommodate those with “no affirmed faith” (from January 2014), of different religious obligations or national allegiances
So, the goddy bits are entirely optional. The question then I guess is really “why is the default opt-in rather than opt-out?”
(EDIT: Sorry for repetition of earlier posts, they all appeared whilst I was reading up on it)