Of course, this isn't that extreme, but you get my point. Spying is justified, dependent on the case.
I already administrate the computer, I haven't spied on him since about 2 years ago or so, but I don't say no when the tool can come in handy. And it probably will at some point. I know all accounts don't need a password. But for e.g.
My mum needs to use the scanner, she tells my brother to do it, i'm not at home, she can't access it because the computer denies the scanner access in standard mode, it needs administration mode.
No I can't teach my mum how to use it, it just won't work, she probably will eventually tell me that she can't find the logout button. I would have thought of all of this already. It's because of my parents that I'm restrained about what actions I can do on the computer.