That's not really true. The providers in .NET are similar to the convention over config philosophy present in RoR. IIS and ASP.NET provide a lot of features in what they consider the "most common" manner. However, almost every one of the built-in providers can be overridden with a custom provider. In fact, I've set Wordpress up on IIS a few times and used a custom ISAPI module to allow mod_rewite like functionality to create friendly URLS.

You don't have to rely on the built-in providers any more than and RoR app has to use only the scaffolded views.

Also, the Mono project has made it possible to run almost any .NET app on Linux. There are a handful of projects at Novell that are written in C#, but are served via Apache on a Linux box.