I wondered this too for a while. I thought that 
dealers did this to help tag "image theifs" and 
such. if they cared, that is. I used to scan all 
my cels with a beige background to go with the 
layout and color scheme of my site. It also 
worked well as a non-obvious "watermark" type 
thing. Maybe the japanese just want MORE color in 
thier cel books? ^^

~Tara