Someone already pointed this out in another thread: what if someone hacks it, starts displaying shock porn. Closing your eyes won't help, neither can you take them off as easy as glasses.
Until someone develops active (LED-based) contacts with high enough resolution to really shock you, that's not a great risk.
I think the current research front is at around 2^8 or 2^9 pixels, perhaps. I'd feel pretty safe from being "eye-raped" by anything on such a display. Note that I mean total pixels, not "pixels squared", i.e. a couple of hundred pixels total.
Of course, a big always-on or annoyingly flashing block of color in your field of vision even with your eyes closed would be pretty annoying. But only until you've managed to take out the contact, which is not all that hard to do with some practice.
How many times on your computer have you been targeted by a hacker who showed you shock porn ? If the answer is nil, why do you think it will happen on your glass?
If we manage to put hight resolution active displays in contact lenses, I'm pretty sure we can manage to implement a convenient safety switch (blink eyes three times? I'm sure somebody will come with something better)
If somebody is going to do malicious hacking on glass or a glass-like device, I would say that displaying offensive material is among the least of our worries.
If the technology were there to have a workable version of Glass as a contact lens, I'd wear it all day every day. Augmentation of the senses is the next logical step, right?