OP I think the main difference between the scenario you're talking about and SBR comes into play when you said: "When his site posted a banner and sponsored a shady book or a book that caused posters to question, on many occasions, he would personally guarantee payment from that book up to a certain dollar amount (a lot of times up to $10K)"
How many "shady" books "or books that caused posters to question" are on SBR's recommended list? Your idea of charging advertisers higher rates because of the Shrink guarantee is a noble one, but how many of those 5 books above would be willing to pay a rate that would be commensurate to guaranteeing the balances of every SBR poster, especially considering the events that have happened this year? Those books don't need to pay exorbitant advertising rates... they're great books on their own, with or without a grandiose marketing budget. The only people who would be willing to agree to that rate would be those shady books you referred to because they would be hoping to use SBR to gain credibility in the market and that's not whom with SBR chooses to do business.
It's very easy to offer guarantees you have no intention of paying on dodgy books that the majority of posters will never even bother with, let alone cash out from. But it's a bit far-fetched to think that at a forum with 130k + members, SBR can guarantee the balances of every poster that is playing with a recommended book.
*Just my opinion. Not speaking as an SBR employee.