The difference is who controls it. I've run a tool on my machine that recorded every key press because I was researching how to optimize keyboard layouts. If someone else did that, it would be a keylogger and it would be very much not okay. But since I did it intentionally, it was a useful tool - though, granted, short lived because it made me nervous having that data around even in encrypted form. There's something there about tradeoffs and informed consent.