>They don't, but I'm confused how that matters.

>I can easily unplug them and insert my own [keyboard/mouse]

Can you see where the problem lies? For you to be able to insert your own kb/mouse, you'd need to bring it from home in the first place. And while it might work if you're at an Internet cafe which isn't super strictly controlled, in an office with more stringent security requirements and checks you'd at least raise some eyebrows.