I'm not in favor of hiding service manuals, but I will mention that in the days of CRT's, one could fry oneself fairly easily. (CRT's can continue to carry significant charge at high voltage, for varying periods of time after being turned off.)

P.S. As a specific example, combine this with the original all-in-one-box Macs' shitty power supplies (and co-located video tuning controls, now that I think of it), and you had a not too uncommon reason for people to go poking around the CRT.