It's because they aren't interested in tolerance or kids, they want to destroy an institution.
I don't think funding will be a problem for Scouting. As local "charities" pull their funding, people will bypass the charities and give directly to the Boy Scouts. I know I did this. The United way can pound sand. Cutting out the middleman means more cash for the local Scout councils.
For the same reason animal rights groups demand veggie items on hamburger chains' menus instead of just starting their own veggie fast food chain -- because they know the public wouldn't go for it, and they would lose a bundle.