It could be an individuality thing. If you're like 98% of other, you're lost in the crowd. But if you're in that *special* two percent, you stand out. Whether this is valid or not...And I think people here are *special*/unique, so they enjoy having it confirmed?
I don't think this question is about thinking outside the box, to me, it's just testing a mostly unconcious reaction. It might make a difference if people knew in advance that they were supposed to think of a red hammer, like the Pick Option A poll question from awhile ago.

Maz"course, could be warped by my disgruntlement at seeing a red hammer"er31