Question: I'm wondering why some people release software as freeware, yet they don't release the source code. Why is that? I can think of some reasons, yet most of them don't make very much sense. Why would you want to keep the source closed but let the program be freely available (free of charge, not free as in freedom)?

Answer: 
One of my favorite productivity tools is freeware. I asked the author about the source one time, and he said he couldn't release it because it contains a lot of proprietary code that belongs to his employer. So I suppose that his employer doesn't mind it being used in a free tool, but that it's also being used in their commercial products and they don't want to give away the code to it.
I have several freeware apps for which I wont provide source code. The main reason is because they share large amounts of code with commercial applications. Consider something like a document viewer... that still needs the rendering system of its commercial cousin, a document creator. Another reason is some of the apps also use non-open sourced 3rd party components.