Most of the time, those players have no clue the capabilities of their opponents, and probably don't even know how to play their champ to the fullest either, and they just think that if they surprise the enemy they will get the upperhand and kill them.
I suck at harrassment, so I tend to be quite the opposite. I will mid as Teemo and let my opponent push me all the way to my tower where I can just pick off the minions with tower help. I hate trying to harrass because I always end up feeding if I do.
Strangely enough, the only champ I feel confident harrassing with is Sona because she is pretty sustainable, and I usually have a partner who like to take advantage of that fact.
--
http://img218.imageshack.us/img218/3156/couchpotatocat.gif