I'm not 100% on this, but I'm pretty sure that this is how it works from what I have seen:
They can't "just clear the pixels they need"
You can see this when you go to the main menu and select with the joystick to underline stuff, there will be faint ghosting under previous items. Also, when you use the cursor with the dictionary to search for words, you can see everywhere the cursor has been previously before you turn the page. It's not very noticeable because it is between words. But imagine turning the page and having this all over the page, in the form of faint words. It could be annoying to some users.
What I have noticed on the DX, after dictionary definition is used, and then you clear it with back, they repop the bottom bar, and the contrast is wrong on the bottom bar gradient, because they only refresh the pixels they need. It would be better to just refresh the whole screen, but whatever, probably saves battery life.
Sure you can set the screen driver to not invert the entire page, and it will display the text, but there will be slight ghosting from previous text that was inverted (similar to using a pencil eraser to erase some text, it does not completely clear it)
The further outward you invert something, the more clear space of ghosting you will have on the screen, so it's easier just to invert the whole thing at once.