Because you can switch the background color to black. In that case all black lines would be invisible, so TBC is inverting them. The same for "White". This is handy if you switch the drawing window background color regularly.
But especially for plotting that can be really annoying if you set the text color to white, i.e. for text on top of an image and since the paper is white it changes the text to black. We can't print actual white for instance, everything past 244,244,244 is converted to black.
It can be mitigated to a certain degree by setting alternative layer colors for separate view-filters in the advanced view-filter settings. But this one is rather cumbersome to use. The window doesn't remember it's size settings, depending on your layer/image/surface name length you may have to resize that window every single time you go in there. And you can set colors only one by one, whereas you can change visible ticks for multiple layers at once. Usual user interface problem in TBC. It's botched together, it works to a certain degree, so no developer will look at it again, but it's cumbersome to use.
This is one of the topics on my ever-growing annoyances/bug list our dealer gets nearly every week from me. There are so many bugs and annoyances when it comes to drafting and plotting.