Having used PayPal to process thousands of transactions for almost 8 years now, I've never once seen PayPal issue a refund for "no valid reason". You will always receive an email outlining why the funds are being placed on hold pending an investigation. You will also have the opportunity to respond to that investigation by providing whatever correspondence, log files, etc. you feel are pertinent to the investigation.

I Have also worked with many other payment providers (2checkout, authorize.net, AlertPay, WorldPay, FastSpring) and they all do the same thing.