The charities sign a contract with the third parties unfortunately - eg they have permission from the charity.

Here in Europe oxfam for example uses some of these private companies and they get the first year of donations and from the 2nd year it goes to oxfam itself.

Apparently the average person cancels donations after 2,5 years so for a zero marketing budget (for oxfam) they make 1.5 year x your donation.

When I first found out I was disgusted and some majors in countries in Europe have tried to ban such "paid charity workers"... (They tend to operate near train stations etc.