Sure its the persons fault for not paying attention. The fact of the matter is that there is no honor in that. Attacking someone when they're unawares is dishonorable in just about every code of honor.
it's not the ninja's fault that the person is unaware and can there for attack them. And stop saying that ALL NINJA'S are like that. Some will let you get the first attack and let you miss and then attack themselves. You cannot say that all ninja's do that because it's not true and I don't like a stereotype like that. If everyone were awarre of the other's attacks, then where would the challenge lie? When two samurai lock blades, one may use a seperate tactic or attack or something that will catch the other off guard. samurai aren't different. Some samurai attack from the sides because they are intent on winning the battle at hand, just like if anyone saw an opening. Sure, some ninjas do it, but so do samurai, so don't say that it's just one sided. If you make an opponent stumble after a blow, would you let him/her recover and have time to plan another possibly stronger attack? Is that wise? NO, it's not and ninjas take advantage of that, whether it be from behind of the side. If it were life and death and you saw an opening to attack, whether it be from behind or the side, would you take it, or allow the opponent time to get ready to kill you?

Please, know the facts and quit saying that all ninjas do is shoot shurikens from treetops, shoot poison darts *which is stupid* and attack from behind, because it's not true