So i understand the basic premise, if you have 51 % of the has power you can create a longer private block chain and then when you release your longer block chain it becomes THE block chain. Allowing you to undo transaction in the shorter chain.

But the question i have is why? Why do clients just accept the longest chain without question? Is there a legitimate reason that entire blocks in the past would change after being added to the chain? Couldn't you program the client to have a "memory" and detect changes in past blocks? Sure it might be CPU time costly but if i was and exchange or payment processor I'm sure the security would be worth it.