SEC does not approve of insider trading after eavesdropping. (Minnesota man finds out the hard way.)
How valuable is conversation? Much of your day to day conversation may need to be kept confidential. Even if you know you are discussing critical, private matters, those around you may not have the same understanding. Eavesdropping resulted in $140,000 in fines for a Minnesota man who eavesdropped on his wife's conference calls.