Snape's a teacher, he could easily have deducted as many points as he liked from Harry and given him a "lifetime's" worth of detention if all he wanted to do was protect the orders business. Moreover his reaction was not one of a calm professional, but more like the emotional and terrified reaction of a cornered and wounded beast. That kind of reaction indicates lack of self confidence and lack of love and belief in one's self. You must posess self confidence and strength to protect a secret. These are weaknesess. Weakness will only invite further intrusion. And why did Harry not therefore attempt to see into Snape's secret thought's again you might ask? Because Harry's the Hero and as the hero of the story he knew to do so would inflict even more unnesecarry pain hurt on Snape. Harry might hate Snape, but not enough to again, cause him to suffer the kind of anguish and hurt that Snape obviously felt when he discovered Harry in his pensieve bowl.

Snape is nothing more than a moral and emotional weakling.