That isn't exactly what I said. I said the law (all over the US, I believe) does not require anyone to try and stop a crime, or even report a crime in progress. I'm not sure why. There are exceptions for certain professionals being obligated to report certain types of crime, and there is also a legal obligation for a husband to protect his family or a parent to protect his or her child. That is how it was explained to me.
Here is a pretty decent article that covers some of that issue:
http://criminal.lawyers.com/crimina...ing-ignoring-falsely-reporting-and-lying.html
Now, if they were directly asked and lied after the fact, that is obstruction. However, often, people won't be charged for that if they cooperate at trial. I didn't say it was pretty, but that's the way the system often works.
Unfortunately, being a slimy dirtbag with no moral compass is not a chargeable offense in Tennessee. And probably not any other state either. If it were, we would probably have to fence off an entire state to use as a prison.