because if you are at 'work' or such environment that warrants the NSFW (Not Safe For Work) tag distinction and you click on a bubbled picture, it would give the illusion of viewing softcore pornography rather than, say, a typical bikini picture from a social networking site (which would arguably not be okay to view at work to being with).

Your mind fills in the gaps. What makes a picture NSFW is what it makes people think, not so much what it actually is.