This blog post from Microsoft explains why Office 2013 and Skydrive work this way
http://blogs.office.com/b/office-next/archive/2012/07/30/office-and-skydrive-for-windows.aspx
This is a summary
Although in theory it sounds good I don't like this behaviour either and so I've disabled this option in the skydrive settings and in the office settings.
- The skydrive settings are accessed via the status icon
- The Office settings are accessed via the FILE > OPTIONS > SAVE dialog as described in the article linked in the comments above
Both changes seem to be required to get back to the way it worked in Office 2010, ie always a transparent background process (but probably less bandwidth efficient)