I work at GoCardless.com, a UK-based payments startup (YC S11), and thus I have a above-average insight into these kinds of issues from my day-to-day work.
From the point of view of fraud protection and risk management, I can see why PayPal take this kind of unilateral action. Payments providers are always at high risk, and since they deal with and indemnify other financial firms further up the chain, they're the ones taking the risk.
Even when you're charging relatively high fees as PayPal does, one case of fraud or even something as simple as a project that doesn't materialise (not that I think that'll happen with Mailpile!) can eat up tonnes of your revenue and make your business unviable.
As such, GoCardless sometimes has to make decisions that account holders won't like. However, the difference between us (as well as, I believe, other providers like Stripe) and PayPal is that we will be (a) reachable and (b) reasonable.
For me, PayPal's failing is in the lack of customer engagement on these issues. Every story like this shows PayPal as cagey, unhelpful and unwilling to have a discussion or reconsider at all. Everyone can, to a degree, understand why PayPal has to make unpopular decisions to protect itself from unacceptable risk (as all businesses will have to do in various ways), but their approach and manner in doing so is what is unacceptable.