Imagine if you're a fraudster, you've knocked up a fake event and you've sold $10k-$20k in tickets, you could withdraw the money and disappear before the date of the event. Now Paypal are on the hook for the money, when they get creditcard charge-backs it comes out of Paypal's account and not the fraudsters.
Does anyone consider it reasonable that Paypal take this risk ? - the reason for the 90 day limit is that's the length of time most consumers have to request a credit card chargeback. Paypal is simply protecting themselves.
If you're engaged in high-risk transactions you're much better off going with a specialist provider (i.e Amiando or Eventbrite in this case) as they have risk fraud models designed to handle the specific risk factors.