Shrink-
Of the books colluding, how many are advertising at the top of the forum?
Others-
Books used to like the transfers because they hoped to take customers (and their money) away from Pinny and other popular books. But, of course, this just sped up 'survival of the fittest". Offering exotic gimmicks like props may have brought in some customers, but with high juice, the props offer little alternative to the low-juice offerings at Pinny.
The other practical reason for the book-to-book transfers are simply to save withdrawal/deposit fees. Even if you are getting players, why eat all those deposit fees, if you can just carry a 10-dime figure with a "friendly competitor" book that you trust? With the right books, it could be a worth while symbiotic relationship.