Would not bother me if I was doing the work the boss pays me for.
Reallly? Because much of the work my company pays me for is confidential--student information, budget including salaries, HR work such as hiring, terminations, and disciplinary actions. So, yeah, it wouldn't just bother me if someone was reading over my shoulder without my permission but it would bother my boss and HR, as well.
It's rude and unprofessional to sneak up behind someone and read over their shoulder.
Agreed--of course, the real question is, why isn't the snooper at his/her workstation and doing the work they are paid to do? That's what I'd be asking them, quite loudly, too!