I think the issue is with Webforms specifically. Webforms abstracts away so much of the way the web works that developers who have spent their careers developing on it never actually understand how the web really works. Concepts like Viewstate hide the realities of web development. The fact that almost all processing takes place on the server-side means they are weak at client-side debugging, etc.

That said, I don't think it's fair to hold the same opinion of ASP.NET MVC developers - as it's essentially the same level of abstraction as Rails.