Those young people who try to accost you on the street - they are agents trying to get you to sign up for direct debits to various charities.
They aren't employed by the charity and I can't remember how much their cut is - but the charity doesn't see as much of your money as you would like.
I guess the charities prefer to get money by whatever means.
Personally I manage to donate through my work to a nominated charity and they match the contribution. I wouldn't dream of signing up to someone who is getting a commission from my donation.