Because, if someone apart from me has the unit, it is most probably stolen. At least the thief will have to have a PC to erase the info before offering it to someone in a pub. Plus it is a wee bit more secure than a label that can be pealed off.
It's easy enough to make your own. Just take any image, resize it to 320x240 then you need to rotate it 90 degrees clockwise and save it as "splash.bmp". Then make sure you have a backup of the original "splash.bmp" on the tomtom before copying your new one over the top of it.