The motivation is they want you to sign up for the paid membership because they trick you into thinking they are a credentialed organization (they’re not) and they’re taking advantage of new writers at the beginning of their career who think editors are going to be impressed with putting IAPWE on their resume , the referral letter, etc. If people like yourself don’t fall for it and give them any money they miss out, but enough people do to make it work with. Running that many Craigslist ads is expensive!