Why? They're both acts of violence that people gloated over. He's not saying "killing this guy is just as bad as lynching innocent black people". He's saying America likes killing, whether it's someone you think should be killed or not.
There's still a pretty big difference between violence against random innocents and violent "revenge" against a powerful force. I'm sure people would also be thrilled if an insurance bigwig got sent to prison, or tarred and feathered, or humiliated in some other way.
