The collectors are paid usually by the number of people they sign up. the charity benifits by a ) having a regular income stream which once started is unlikely to be stopped as the monthly amount is so small, B) having your details on the database so they can regularly harass you for more, C) they can sell on their list of contributors hence if you sign up for say RSCPA you will soon get calls/letters from PDSA, cats protection, Worldwide fund for nature etc At some point it will filter down to the bottom feeders and scammers
People get caught out by the boxes with the do not use my info clauses they are often worded so you have to tick one for them not to send you more info etc and the next one which is about third parties is the opposite so you are more likely to tick both giving them permission