You've never had a customer in your professional setting who didn't pay money for goods and/or services? Yet it was very important for your boss (and therefore you, as a programmer) to service their every whim?

Customers are customers. Whether they're paying or not. Not all customers are worth servicing (even with infinite money offered, "firing a customer" is important to keep the community in check).

But this isn't a situation where the RSYNC maintainer should fire the customer. There's a LOT of backlash to this release. Even if this one particular customer is a bit of an ass, there's plenty of good users in that 90+ comment chain (hundreds now?) where this regression has clearly struck a nerve.