I'm one of those people and am happily using Microsoft products through BizSpark to develop web apps. Yes, I could use python with django or php with mysql (and have in the past); but I can build products better and faster with asp.net/c# (it's what I know and like best). And now that I don't have to pay huge licensing fees before I make a profit I can do that.
Personally, I use both MS and FOSS products extensively. I am a huge fan of Python and it is my primary scripting language, but my main career is as an MS SQL Server DBA, and I think that for many applications (not all) it compares quite favorably with mysql and oracle.
