More usually its a way to impart some interesting, pertinent knowledge without getting the person who told you, or yourself, into trouble. The person who told me was in a position to know, but I'll be damned if i'm going to stick his name out there to be sued. I'm a copper and I met him on a course, if you must know.
Also, any money made by the Tapas 7 off the back of their involvement would be seized of they were ever convicted of an offence connected to her disappearance. They were all doctors/partners of doctors and, personally, I think the lot of them were drugging their kids for a quiet life and Maddie was the unfortunate one who OD'd. By revealing the truth, they'd all be struck off and in no danger of being able to continue the lifestyle in which they were accustomed. The idea of losing everything you've spent your life working for is a strong motivator.
With regard their self publicity, I must admit I'm flummoxed by it too. I'm no psychologist, but I suppose perhaps they've dug too deep a hole in the initial stages to ever appear to have given up. They've also made a fair bit of cash of the back of it, some of which was (until they were caught out, of course) used to pay their mortgage rather than search for Maddie.