How is this related to just conferences? PayPal doesn't have an anti-conference agenda.. There seems to be two things that set PayPal off.
1) Small volume to huge volume in a matter of days. 2) Taking payments for something happening in the future.
Story after story about both of these scenarios.
It seems obvious at this point that PayPal is not a good processor for things like pre-sales or events. It may be annoying but it's a liability for them and they do what they need to minimize it.
PayPal may be attractive because it's quick to setup but it seems easier for people to get in touch with a proper merchant account provider, discuss what you'll be selling ahead of time so they're prepared, and not get your funds frozen.
Update:
For anyone who is interested, here is PayPal's policy on pre-sales.
https://www.paypal-businesscenter.com/content/presale-policy...