From reading many of these stories, it's clear that PayPal wants to cap their risk. For a brick-and-mortar business, their risk is the difference between sales that have been billed and product that has been received by the customer. That might account for a week or two worth of sales volume on a rolling basis, so a business doing $100k a year of PayPal transactions will typically only have $3k-$4k at risk, which is on par with what PayPal will earn by serving such an account over 18-24 months.
For a conference, however, there might be six months of revenue for pre-registration before the actual event takes place. If the event planner then absconds with the money, PayPal could be on the hook for the entire amount, which has long-since been withdrawn and spent. Take that same hypothetical $100k business for a conference, now all $100k of it is at risk, and PayPal only stands to make $2,500 or so off of that.
It's pretty clear that the credit card companies have PayPal by the short-and-curlies, in the sense that PayPal themselves will be on the hook for any chargebacks.
I call this the "wash my Ferrari" problem. If you do a good job, you make $10. If you scratch the paint, you're out $10,000. Not a good risk.
Open question to HN: how could PayPal do a better job of handling the situation?