These organizations are FOR PROFIT companies collecting for charities. Charities pay them to collect for them but in the end the charity gets very little from the calling company. By using a charity, they get around the do not call lists.
One of the best ways to really annoy these people is to say: "Thank you for reminding me to make a donation. I will call the local chapter of the American Cancer Society right now and make a donation to them directly." (the telemarkers really hate that and will then try to convince you to give to them instead. Say "Thank you for you help and goodbye" and then hang up.)