Not everybody who uses Mongrel is a multimillion-dollar company.
Yet they can afford to hire employees who know how to deploy Mongrel and Rails. Programmers and sys-admins aren't cheap, neither are executives, marketers, salespeople, etc. Even if you're running a tiny Web startup, you should be able to afford small donations.
edit: You can treat the donation as a "license fee" if that makes the accounting easier to deal with
Maybe if it wasn't free, nobody would be using Rails and Mongrel - everybody would be on C# and Mono. There is always competition, so "should be rewarded" is really determined by the market. You can't just work on anything and then demand money for it.
Mongrel that costs money would be a different product from Mongrel that is free.