Thanks for providing a useful bit of information. I've passed this along to my wife who scans lots of old family photos for her scrapbooking. Just to speculate as to why a white document mat is used, I'd guess that since most scanning is of white paper documents, this is the best choice to both minimize visibility of the document edges (in scans, copies, etc.) and to least effect the autoexposure function, again assuming mostly white paper documents as the sources.
I'll have to try a black background for scanning a printed on both sides, semitransparent (thin) paper document to see if it reduces the "bleed through" from the opposite side. I'm guessing it will, but might also effect the autoexposure output. We'll see.