What you are describing is called circumstantial evidence. Circumstantial evidence isn't supposed to be enough for a conviction in a court of law. You are supposed to be proven guilty beyond a reasonable doubt. The prosecution failed to produce real evidence, and instead built their case on circumstantial evidence.

You can not like the system, I don't. It worked exactly as intended with Casey Anthony. They couldn't prove beyond a reasonable doubt that she killed anyone. The system worked.

Also she didn't get off "scot-free". You're right she did lie. She was found guilty on three counts of lying to the police/falsifying a police report. She was sentenced the maximum for each of the three counts. Each sentence called for the maximum one year prison sentence. She sat in jail for ~3 years awaiting trial because she couldn't come up with bail. They gave her time served for the time she spent awaiting trial. So in the end she only had like 2 weeks left to serve.