Hi Kyle - That's the way the description editor is set up. When your text gets to the end of the box it will word-wrap - it will automatically drop down one space and return. It doesn't let you to do a manual single-space return because paragraphs automatically adjust their width to various templates and configurations. Manual single-space return commands combined with the word-wrapping would result in lines of text of unpredictable length. Depending on the final paragraph width, you could even end up with a line only one word long. Pretty bogus looking.
So the return key is only for when you want to start a new paragraph, thus the double space.