They have been doing so for many years-decades. I was a GS in the 70s and at one point had a semi-openly gay leader. She took us camping and never molested a single one of us. At least 2 male teachers from the middle school I was attending during those years have been convicted of sexually molesting girls from their classes, however.
My thing with the Boy Scout ban is not so much with their ban on having gay staff as it is with a ban on gay scouts. When a boy becomes a Cub Scout, he doesn't necessarily know he's gay at that age. I think it's wrong to kick him out at some future point in time, even if his behavior within BSA and the community is above reproach, simply because he is attracted to other boys.