And 8 interviews? Was it even the last potential interview? That is indeed fubar. They do this because they can. Hiring is an expensive process, and not everyone can afford to dump this much effort into a single hire.
Pretty standard across these AmaGoogFaceSoft like companies. They basically have too much money and time to spend on these things, and they inevitable hire people like them who have lot of free time on their hands to prepare for their textbook puzzle questions.
In other words they wish to hire candidates who do bare minimum work but spend hours preparing for interviews to move to next jobs.