While I don't approve of PayPal freezing payment's I can kind of get it with a conference which may or may not take place. They need to protect themselves from having to hand out potentially thousands of dollars in compensation.
What I don't get is why they would freeze an account getting donations. It is a donation. You are not paying for a service which may or may not arrive. Seem's very very strange.
Even if they are bonafide donations, if there are expectations attached to those funds (that the company is going to build a product or perform a specific service, or put it towards a certain cause) and there's a risk those expectations won't be met, then there's a risk for PayPal that some of those donors are going to be upset and do something about it -- like cry fraud and charge back the payments. PayPal has to carefully manage its chargeback rate like every other merchant.